Drop-In-Drop-Out Multiplayer

"Come on, Blue Toad! We don't have all day."

[Shaun sits down and picks up a controller]
Announcer: Player Two has entered the game!
Ed: Ain't you got work?
Announcer: Player Two has left the game!
[Shaun puts down the controller and walks off]

— Shaun of the Dead

In most video games, multiplayer is a matter of choosing the number of players at the start, and then getting a game going. This can be a problem if someone wants to join later. For example, have you ever been playing a game alone when your sibling suddenly walks into the room and wants to play too?

The solution to that problem is Drop-In-Drop-Out Multiplayer. The ability to join at any time, and just as crucial, the ability to leave at any time (which helps you avoid that annoying problem where your little sibling suddenly doesn't want to play anymore, leaving you with a second character just standing there). These titles have no real distinction between single and multiple-player games, since those initiated as one type can easily turn into the other and back.

This solves more than just the sibling problem — it's also is what makes games with large player limits practical. The Game Lobby system, which requires players to get together ahead of time in a group and start the game all at once, would simply never allow for such games to happen by itself.

Since this means remaining playable with a fluctuating number of players, such games are usually either Co-Op Multiplayer or heavily reliant on bots.

---

Examples:

General

• Coin-op arcade machines (and many franchises originating from them like Gauntlet) will traditionally have “PLAYER 2 INSERT COIN” flashing perpetually over the upper right corner of the screen where P2's HUD goes, so that just dropping a bit of lucre into the corresponding slot will let any passerby join the game. This is especially useful with titles like the 6-player X Men arcade game. It's generally wise to ask players before joining in, out of courtesy.
  • Time Crisis 2 and its successors allow you to lock out other players by selecting the Solo Play mode.
  • Darius Burst Another Chronicle supports up to 4 players, and you can set a message at the beginning of the game to "request support" (that is, invite other players to join). You can also put the game into "Private Mode" to disallow new players.
• Ubiquitous in competitive online games, if for no other reason than to account for the possibility of some players experiencing connection problems in the early days. Now because, especially with Massively Multiplayer games having thousands of players in one level, and even many “normal” 100+ player games like Battlefield or Enemy Territory , it would be almost impossible to get everyone together for a session in unison (though clan and LAN players try valiantly.)
• Games not particularly suited to this sometimes include aspects of it to deal with shortsighted players, especially games that tend toward lengthy matches. It's quite common, for instance, to see a larger Battle for Wesnoth game where half the factions have been handed over to remaining players, the computer, or spectators.

Other

• Civilization 4 had this, though it was a real pain coming into an AI civ where you didn't know where all your units were and had to immediately fortify them all.
• Dance Central: In the second, third, and fourth games, a second player can join in or drop out at any time by standing in front of (or moving out of the way of) the Kinect.
• Dark Souls gives players the option to briefly pop into another player's world as a phantom, either to assist them (which lasts until either the phantom leaves, the player defeats the boss of the area they're in, or either of them dies) or to invade and kill them. The brief nature of these excursions means players don't have to devote a large chunk of time to the multiplayer aspect. On the downside, it also means the game has no pause function.
• Deep Rock Galactic lets players join other players mid-mission. In this case, the "drop-in" is literal; the joining player arrives in a personal drop pod that drills through the cave ceiling and lands next to the rest of the team.
• Diablo is almost MMORPG-like, with players able to join and leave at will, form into parties, and the game even applying Dynamic Difficulty to compensate for the extra players.
• Dragon's Crown, being a Genre Throwback to the High Fantasy Beat 'em Up arcade games of old, naturally employs this. You could even set it so that AI-controlled characters could randomly join during a dungeon crawl if there's still an available player slot.
• The End Times: Vermintide and Vermintide II are First-Person Shooters that always have a team of four; players can join or drop out during the level, with bots replacing any missing humans.
• In online versions of Everybody Edits, players can join and leave levels at any time as long as they're public.
• Fortune Street has the "Out To Lunch" option, where a player can take a break from playing and a computer AI will take over until the player opts back in. Notably, earning points allows you to purchase personality traits and roles to assign to your Mii that also determine the AI's general strategy. This is an unusual form of drop-in, drop-out in which existing players can leave and return, as opposed to new players joining.
• Gears of War didn't have this at all. 2 added it to campaign co-op (with a bot replacing Dom). 3 added it to campaign co-op (with bots replacing your other 3 party members) as well as versus multiplayer. It is also possible to join a Beast or Horde game mid-stream, but it requires joining a friend's game through the X-Box menu.
• The Guitar Hero series introduced Party Play in Guitar Hero 5, which allowed players to play as many songs as they like with the option of any player to pick any instrument and drop in when they like. When players join, the interface adjusts to fit more players on the screen, and when players leave, their note chart disappears.
• Kirby's Return to Dream Land allows players to drop in and out during levels. They can simply press the + button to join, or push and hold the - button to drop out.
  • Similarly for playing as Helper in the SNES version of Kirby Super Star, except the game wouldn't take control of Helper until the second player's character died and was revived by the first.
• Left 4 Dead has four characters at all times, regardless of number of players. Joining the game means controlling one of them, leaving means switching them back to AI control.
• The Lego Adaptation Games do this. They typically have a second character following the first player around, and any player who joins instantly takes control of the second character. Quitting the game means that character becomes AI-controlled again.
• If a local player turns on a controller in LittleBigPlanet, the game asks them what account they want to log in as, and once they pick, they spawn out of the next checkpoint. Online players asking to joins prompts the game to ask if you want them to join, using Triangle for yes and Circle for no. Local players can turn off their controller, and their Sackboy will sit down and leave the game after about 10 seconds. Online players can just leave, or be kicked if you so desire.
• Mario Party games allow you to set human players to computer players and vice versa at any time except during cutscenes and minigames. This is also useful for getting the coins and stars collected by computers at the end of the game.
• MechWarrior Living Legends allows players to enter a match at any point. The progression-based gameplay means that a player joining halfway through the (usually 30 to 80 minutes) game may have to fight heavy battlemechs while in a basic light mech, but the Comeback Mechanic introduced in the final update and players giving money to each other can get someone up to speed quickly.
• In MindJack if you allow your game to be open other players can "hack" into your single player game, to help or to hinder.
• One of the biggest overhauls new to Monster Hunter: World, the fifth entry in the Monster Hunter series, is the addition of the SOS Flare system. If you're having trouble against a tough monster, send out an SOS Flare to invite other players into your quest.
• Starting with New Super Mario Bros. Wii, Nintendo's new retro games including Kirby's Epic Yarn and Donkey Kong Country Returns take the approach of allowing players to join or leave only inbetween levels, but at least it's better than having to set the number of players ahead of time and have it be fixed. The Wii U sequel, New Super Mario Bros. U, allows players to join mid-game during a level, but not necessarily drop out mid-level.
• Normally played straight in Overwatch, but averted in Competitive Mode, where players that leave, voluntarily or otherwise, cannot be substituted. If the cause is a disconnect, they have 30 seconds to reconnect before they're booted from the match altogether, and even then they can only reconnect up to two times. Additionally, if they don't come back, players are allowed to leave with a loss but without quit penalties, as a team that's down by one (out of six) is probably going to lose anyway.
• A lobby host in PAYDAY 2 can allow or disallow both this (the drop-in part, at least) and AI-controlled allies.
• Rock Band 3 introduced the ability for players to drop in/drop out at anytime, even during mid-song.
• Sin and Punishment: Star Successor, after a fashion. Since Player 2 is simply an on-screen cursor and cannot take damage, and there is no Multiplayer Difficulty Spike, the second player can stop playing at any time without worrying about causing serious problems for the first player.
• Playing as Sonic and Tails in Sonic the Hedgehog 2 or Sonic 3 & Knuckles, a player could grab the second game pad at any point to control Tails, while if there wasn't any input for about 30 seconds, the game will start controlling Tails again.
• Star Ruler and Star Ruler 2 allow players to take over AI empires. If a player drops out, the AI takes over, but the player can return and take over the AI's control.
• Sword of the Stars even allows players dropping out to leave the AI some basic instructions on how to run their empire and password lock their slot, in case they intend to come back and retake command later.
• The Tales Series usually enables this. Pause the game to set player 2 as controllable, then play. If you so choose, you can even make every character controlled by the AI.
• Team Fortress 2: allows players to come and go; some servers will even populate "empty" slots with AI bots so that even a handful of humans can still play a (somewhat) meaningful round. In addition, an autobalance and team scramble system is kept in place so that no one team is ever severely handicapped by mass quitting or one side repeatedly curb-stomping the other. Some servers disable one or both of these aforementioned systems, which plays hell with game balance as one side is left with only 4 or 5 players while the other has 12 or more, leading to a curbstomp battle with no one wanting to join the losing side.
• The Warriors allowed a 2nd player to control another Warrior character any time they jumped in and when they drop out, the character goes back to AI control.
• World in Conflict is one of the few online RTS games that has this, made possible by the fact that it has no base building, so new players join at any time during a match without any gameplay disadvantage compared to players who've been there from the start. They do have the disadvantage of having less time to score points and the game also penalizes you for joining the already winning team if there are vacancies in the losing team (reducing the maximum amount of units you can field at once).

---