Whenever it's time for a musical number, the lead dancer and singer (whether hero or villain) can usually pull any Innocent Bystanders into a Crowd Song, and thanks to Spontaneous Choreography have them all dance and sing in perfect sync.
Some leads can go a step further. Rather than make do with whoever is nearby, or go solo if there's no one else around, they can Summon Backup Dancers from out of sight, bringing a battalion of ballerinas to dance at their side. To make this more impressive, they'll usually be summoned directly from the Universe of Wardrobe and Costuming after having been fitted in perfectly appropriate attire for the song in question.
This trope isn't usually a magical spell that can summon dancers like a wizard can summon monsters (though in a wacky enough series, it may well explicitly be one), more often it's an implicit rule of the cosmos. Anyone who goes into a musical number will get accompaniment, even if it requires the Spontaneous Generation of said dancers from hard vacuum. Other times, it can be humorously Justified by the lead in question training friends, allies or subordinates beforehand and instructing them to hide out of sight and march out on cue for their big song.
If this is a magical ability of some type, the backup dancers may help out by dance battling alongside the summoner.
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Anime and Manga
Extra ballerinas appear out of nowhere from time to time in Princess Tutu.
In Scott Pilgrim, Evil Ex #1 Matt Patel can summon Demon Hipster Chicks to help him cast his flaming hand attack. In the videogame, these attack the player.
In Shrek 2, the Fairy Godmother enchanted Fiona's furniture to sing along with her about giving Fiona a makeover.
Giselle's Disney-Princess magic in Enchanted turns the jaded denizens of New York into her backup chorus, leaving her stodgy love-interest baffled that everyone seems to know the same song. "I've never heard this song!"
In The Mask, the titular character does this to a SWAT team.
In The Blues Brothers, when Ray Charles plays "Shake a Tail Feather", passersby in the street start dancing, and the crowd quickly grows as people rush in from all sides. They proceed to demonstrate the different 1960s dance styles that are mentioned in the lyrics.
You see those people on the elevated train station way in the back? They weren't involved in the shoot. They were waiting for a train, saw what was going on, and joined in.
Played straight earlier when Tara is singing her love song to Willow "Under Your Spell". While dancing through the park she passes near two other girls resting by the pond who are compelled to rise to their feet and start dancing behind her.
Ryutaros from Kamen Rider Den-O not only has a summonable group of breakdancers (one of whom was the Victim of the Week for Ryuta's spotlight arc), but his powers include the ability to make everyone nearby dance along with his theme music.
In The Movie (the first one), he even summons a kick line of chorus girls.
Almost too obvious an example is Glee. They don't do it all the time, since they actually have cast that are backup dancers, but there was a scene in the outdoor cafeteria that did that, and one in a mall, where bystanders joined in the action.
Most notable are the Troubletones dancers who appeared in Chicago without riding in the team bus (which had plenty of room to spare for them).
In No Good Gofers, the "Dance Party" mode features additional gofers appearing to dance in sync with Bud and Buzz.
Averted in the 1993 Broadway version of Tommy, in accordance with composer Pete Townshend's strict instructions to collaborator Des McAnuff that there be "no fucking dancing."
Pippin does this most obviously in "Kind of Woman," which brings a bunch of adoring backup singers come to Catherine's side. They disappear when the song is over, leaving her alone with Pippin as before (but not really, since the show has No Fourth Wall).
This makes up the first phase of the fight with Armaros in El Shaddai.
in Final Fantasy X-2 The first cinematic is of a concert with back-up dancers that appear via magic/hologram/goons.
Any dance number Phantom R has in Rhythm Thief & the Emperor's Treasure will feature two clones of Phantom R in all-black clothing dancing behind him, which the game never explains. Bizarrely, Jerome, an impostor Phantom R, also receives two such clones.
There was a Sinfest strip where Slick and Co. tried to face off against a G-Man in a dance-battle - but when they were done, he just snapped his fingers to summon two other, similarly-dressed guys to act as his backup-dancers for a modified rendition of Here Come The Men In Black.
Ink City: When Yakko and Rigby break into their The Villain Sucks Song "Pretty Fly (For a Toon Guy)", the first thing Yakko does is summon some random backup singers/dancers. (Trevor is completely distracted by this, and tries to charm the girls into defecting to his side, to no avail.)
On The LXD, Sp3cimen has this power. When fighting other dancers, he can summon around four backup dancers who help him power up his Ra blasts and provide cool visual effects.
Backup dancers sometimes appear if Doofenshirmitz tells Perry this week's backstory in song; he explicitly hired them at least twice. The trope is sometimes justified only by Rule OfDon't Be So Pedantic, It's A Cartoon, such as when Baljeet sings about his fear a bad grade if he fails to make a portal to Mars - he summons not just backup dancers, but scenery and elephants too!
An episode of The Boondocks shows us Tom pining over Sara after she throws him out of the house. He then breaks into an Usher-style love song in the middle of the street, complete with back-up dancers who appear to be just neighborhood guys in their robes and pajamas. They perform perfectly until they are almost hit by a car.