Multiplayer: Drop-in

This is mainly for campaign/storymode gameplay where additional players can join at any point without having to wait for the mission, match, or such to end.

We can somewhat safely assume that all the non-story/campaign multiplayer modes allow this always (except for strategy, survival and LMS games) and should therefore be ignored.

If this is limited to people who were initially in the game, see rejoin, as this tag is for instances where anyone can join at any time and not limited to people who originally were in the game.