In the Fullmetal Alchemist manga, Roy's group has a scene where each of them is shown displaying what their specialty is, culminating in everyone being on board with the long hard slog that is being the rebel group in a corrupt military.
Voltron, (and, obviously, its original incarnation, Go Lion) both Lions and Vehicle Force versions, practically defines this trope, particularly Lion which showed each Lion emerging from its specific hiding place to enter battle. The hiding places reflected the powers and abilities of their respective Lions (Red emerging from a Volcano, Blue from a lake etc.) While it is true that the Voltron team members themselves are usually in the same place when this happens, they must immediately separate to get to their Lion or Vehicle before coming together again. It's a little different in Vehicle Voltron as the main team is separated into smaller teams that are always together in getting to their vehicles.
In Fantastic Four #1, Reed Richards first summons the group together by creating a huge cloud above Manhattan that bears the words "Fantastic Four" before morphing into a "4." The other three show off their abilities because it's symbolic or something: The Invisible Girl vanishes in public, and being quite a ways away has to take a cab while invisible. (This actually works, though it scares the pants off the driver.) The Thing ditches his disguise, causing traffic accidents and drawing fire from the NYPD before he opts for the sewers. The Human Torch flames inside the car he was fixing, melting it, and while in flight is intercepted by jet fighters and ultimately a nuclear missile — still over Manhattan — requiring Reed to use his stretchiness to save the day.
This occurs after the team has been assembled in the first issue of the next-to-most-recent Suicide Squad, where the Terrible Trio Injustice League is put to work doing dangerous missions for the government. As they land on the island their mission is to take place on, it becomes clear what everyone's role is quickly: Big Sir hauls a gigantic watercraft on his back with ease, Clock King calculates the exact amount of time the task will take, Major Disaster barks orders and coordinates the team, Multi-Man frets about what his ever-changing powers are right now, and Cluemaster proves to be astonishingly perceptive.
The Teen Titans in the comic book had an Avengers Assemble (or rather, Titans Together) gathering. When the Justice League refused to help Raven with her demon father, thinking it a trap, she appeared in the dreams of various teen heroes and rallied them to help her fight Trigon. Starfire just so happened to be escaping to Earth from her captors in time to help them.
Marvel: Contest of Champions does this with pretty much every living hero who existed at the time in the Marvel Universe, plus a few new ones they spotted on the way. Though instead of a call, they were teleported (without choice) by a weird red light.
Showed up in a few issues of G.I. Joe, like issue #2.
Appeared in Issue #28 of The Brave and the Bold, the debut of the Justice League of America, where Aquaman learns of a monster called Starro and uses his belt to signal the rest of the League for an emergency meeting. Most of the remaining Big Seven responds, as is typical of the trope (Wonder Woman breaks a date with Steve Trevor, Green Lantern sets his test plane on autopilot, The Flash takes care of a tornado, and the Martian Manhunter had just received vacation time for his human guise when he got the call), though Superman and Batman had commitments too important for them to abandon (Superman is fighting off a meteor storm about to enter the planet's atmosphere, and Batman is overwhelmed by a crime wave in Gotham City).
The four original Defenders — Strange, the Silver Surfer, Prince Namor the Sub-Mariner and the Incredible Hulk — were afflicted temporarily with a plot device in form of a curse that would "assemble" them for every major threat, whether they wanted to be there or not. They were freed at last when it proved to be the work of an enemy who was feeding on their anger and resentment.
Just about everyone looks up from whatever they're doing to see soldiers and resigns themselves to the inevitable. Affleck is so smug at Willis swallowing his pride to come see him personally that he comes without(much) complaint. Michael Clarke Duncan starts up a Chase Scene, yelling "COME AND GET PAPA BEAR!"
Tweaked a bit in Star Trek III: The Search for Spock. The crew needs no introduction; nevertheless, following the scene in which Kirk decides he's going to disobey Starfleet and steal the Enterprise, we get a Mission: Impossible-esque montage of each crew member doing their part to execute the heist. Each person's part is related to their skill. Kirk breaks out McCoy, Scotty sabotages the Excelsior's engines, Sulu kicks a guard's butt, and Uhura famously shows a cadet "some adventure".
Seven Samurai may be the Trope Maker. The leader of the eponymous warriors is recruited by some villagers to protect a village from bandits, and he goes about convincing six others to join him.
Its American adaptation, The Magnificent Seven, has Steve McQueen's character Vin hold up fingers to count the members of their group when each new man joins.
In The Bourne Identity, Treadstone headquarters orders all their field agents to go active. Cue the montage of each agent in the middle of some civilian activity, and dropping it upon receiving instructions from HQ.
The Guns of Navarone: The officers planning the operation have a Mission: Impossible-style set of photographs of the soldiers who will be taking part, and list their specialties (lucky, genius with explosives, mechanical expert, born killer etc.)
This happens in Eddie and the Cruisers 2: Eddie Lives!, while Eddie is assembling a new band; We get to see each prospective member play to get a feel for their, you know, style. But, like everything else in the movie, it's retarded; Among other things, Eddie picks a repressed concert pianist to play keyboard in his blues rock band, and also gives a spot to a guy he absolutely hates for no real reason.
The original script for Monty Python and the Holy Grail had this as a sequence, but it was shortened greatly in the final film, probably due to budget constraints. Arthur finds Galahad building a chicken coop for an elderly couple; there were (would have been?) similar scenes for each of the knights.
Stanley Kubrick's The Killing has a couple scenes along these lines; although the main heist team is already assembled, ringleader Johnny Clay is shown recruiting sniper Nikkie Arcane (Timothy Carey) and Maurice, a chess-playing wrestler.
The Sting showed Paul Newman reassemble his old gang of cons in a wordless montage set to Scott Joplin's "Pineapple Rag" where he finds and signals to each by flicking the side of his nose.
The 1971 version of The Andromeda Strain uses this trope for the scientists near the beginning, although it takes a while to get them in the same room because two of the scientists take a detour to investigate the plagued town.
Within The Avengers itself, Hawkeye is the first to meet Loki and gets brainwashed, Natasha is pretending to be held hostage while tricking her "interrogator" into spilling everything, Bruce Banner is in India helping the poor, Steve is taking out his fish-out-of-water issues on a series of heavy punching bags, Tony is putting the finishing touches on his new clean-energy Stark Tower, and Thor shows up last to crash Steve's and Tony's attempt to apprehend Loki.
In Fast Five, Dom and Brian bring together a dream team made up of characters from the past few films, describing what they will bring to the table in a montage.
"The Professionals" includes a sequence where each character is named and briefly described in voice-over while being seen doing what they do.
Played with in The Three Musketeers. Losing his companions en route of a dangerous mission, D'Artagnan must spend three chapters collecting them back up and extricating them from the situations their particular personality quirks have gotten them into.
In William Gibson's Neuromancer, Armitage gathers a team of specialists for his mission: to unite the artificial-intelligence entities Wintermute and Neuromancer.
Mission: Impossible had an interesting take on this, at least in the earlier seasons: The Captain would take out a dossier full of potential team members, many of them shown engaging in activities relevant to their particular skills, and we would watch him picking out the team he wanted — usually the same core members, but with an occasional addition.
The premiere of Hustle spent about ten minutes doing this for the four team members, and was narrated by a policeman explaining their enemy to a colleague. The footage from this sequence was used in quite a few TV spots.
A great example begins the second episode of Hoolywood East TV's New Kids on the Rock.
Leverage does this twice. Once, in the second episode, it showed the team members in the midst of various solo jobs, dropping what they were doing (Eliot had a gun pointed at him and took the guy out, Parker was hanging from a ceiling during a heist, Sophie is attempting to get a job on a soap commerical) to answer their phones.
The failed pilot for a parody of Mission: Impossible, Inside O.U.T. had a such a sequence for members of the Office of Unusual Tasks. The most memorable was the agent whose cover was as a civilian flying instructor. When he got the call, he told his student something along the lines of, "You're going to solo a bit ahead of schedule," and bailed out of the plane.
The Doctor Who episode "A Good Man Goes to War" has a sequence of the Doctor rounding up his allies for a mission. There are quick scenes of them all going about their business (nursing soldiers on a battlefield, KILLING JACK THE RIPPER, etc.), only to come home and discover a big blue box on their doorstep. The second episode of the same season begins with 3 of the main characters being killed one-by-one by another character. Only afterwards do we find out this was all an elaborate plan to gather the team together.
"The Stolen Earth" and the first section of "Journey's End" function as this too, as Rose, Martha, Jack, Gwen, Ianto, Sarah Jane and Luke, Mickey, and Jackie come together into the Doctor's "secret army"
The West Wing does this in the very first episode. Toby, C.J., Josh and Sam each appears in a short character-establishing scene, receiving a text message at the end that says, "POTUS in a bicycle accident." The final scene reveals that POTUS stands for "President of the United States," and that the characters we've just seen all work for him.
Parodied in Scrubs with Ted's band the Worthless Peons. Whenever he yells "Peons assemble!" the other members of the band sprint in from all directions and stand with him. Ted notes that they have to perpetually be within earshot so that they can assemble at the drop of a hat.
In an analysis of the 2012 Democratic National Convention, the BBC's USA political editor Mark Mardell compared the DNC to one of these, with Obama as The Hero, gathering the old gang (i.e, the Democrat's core vote) and the swing states together for one last hurrah.
The introduction videos in the Sly Cooper video games have these, such as Bently: The Brains, Murray: The Brawn and so forth.
A heroic, galactic-scale Big Damn Heroes version of this trope provides much of the plot for Mass Effect 2, with The Con or The Caper in this case being a Suicide Mission through a relay from whom no-one has returned against an unknown alien threat. Lovely. Most of the game is actually an extended Avengers Assemble sequence, with each character getting a recruitment mission (except the ones you start with or download) and a loyalty mission that play to their specialties (e.g. Garrus the Friendly Sniper has a recruitment mission involving holding a fortified position and a loyalty mission that involves lots of foes that are particularly vulnerable to his tech powers, Tali's recruitment and loyalty missions both involve fighting Geth that are particularly vulnerable to her epic hacking skills, Mordin's recruitment and loyalty missions involve fighting Vorcha and Krogan that fall easily to massed Incinerate powers, etc.).
Basically the entire point of Mass Effect 3: Citadel. Shepard brings together his/her entire (surviving) team from the first two games to assist him/ber in taking out an unknown group with designs to end his/her life.
Vega: What about the rest of the crew? Shepard: All hands on deck for this one.
Capture the Point and Payload maps have one or both teams assemble in wait behind an impenetrable door until the round starts.
"Meet the Soldier" seems to take place during a Rousing Speech for an assembly, but then the camera angle switches and we see that The Soldier has actually been lecturing the severed heads of his enemies.
"Meet the Spy" have the BLU Soldier, Spy, Heavy, and Scout (or so we're led to believe) assembling in the Intelligence room to discuss the fact that a RED Spy is in the base.
"Meet the Medic" is a two-man assembly, but since these two men are the Medic and the Heavy, much ass-kicking ensues.
In the promotion video for Mann vs. Machine mode, what's implied to be left of RED and BLU assemble to battle the robots.
In the RuneScape quest While Guthix Sleeps, the player must assemble a group of 8 warriors, slayer masters and legendary adventurers in order to take on the Big Bad, which results in the player traipsing round Gielinor to recruit the warriors.
The Animated SeriesM.A.S.K. did this. Once an Episode, a computer would review the specifics of the upcoming mission and select the appropriate operatives for the job based on their helmets (Masks) which gave them their powers, their natural skill sets, and their vehicles.
In the Teen Titans episode "Calling All Titans," Robin contacts every single member — and there are about thirty! — to let them know to stand by for further instructions.
The first episode of Transformers: Robots In Disguise has the Autobots issue a battle protocol, and Optimus Prime requests that the Autobot Brothers be sent in to stop the Predacons. T-AI goes over the Autobot Brothers' abilities and personalities.
The teaser for The Avengers: Earth's Mightiest Heroes shows the trope used along with the Trope Namer: When Iron Man calls "Avengers Assemble!" and activates a signal, each Avenger is doing something until hearing the signal; Captain America is training, Thor is meditating on the sky, and Giant-Man and the Wasp are making a son (they are building Ultron, you dirty-minded!)
In the eighth episode of the show, we learn that the Avengers order each other to gather with communicators that double as ID cards.
Jackie Chan Adventures does this whenever Jade calls together the J-Team, but it's only a two person montage since Viper and El Toro are the only ones not in Uncle's shop/Section 13 at the time. And of course, when they arrive Jackie tells them that Jade wasn't supposed to have called them and they should just go home.
Rise Of The Guardians has an epic scene at the beginning in which Santa calls the other Guardians, consisting of the Easter Bunny, the Sandman and the Tooth Fairy, in order to fight the Boogeyman.
Darkwing Duck has a reference to this, in the "Just Us Justice Ducks" two parter, Darkwing rallies his newly formed team for the final battle against the Fearsome Five with "Justice Ducks! ASSEMBLE!. Evil Counterpart Nega Duck just yells "Fearsome Five get over here!"
One episode of "Young Justice" has Klarion the Witchboy summoning various evil magicians for a scheme. Extra style points for bringing them in on the corners of a big pentagram.