Fortnite Battle Royale is an example of a non-continuous multiplayer game. 100 players join a game which has fixed start and end times. When there's only one player remaining, the game ends.
Agar.io is an example of what I like to call a continuous multiplayer game. Players join in and die out, but the game goes on forever.
How would a game developer design a system to split players into different groups when there're too many online to fit in a single game? Let's say a game has a limit of 10 players, and 50 players are trying to join. The server will split them up into 5 separate groups. New players will be sorted evenly into these groups to replace players who died. When the player count increases past the global limit, new games will be created to fit them. The problem arises when the player count decreases. The system needs some way to seamlessly delete extra groups of players that are no longer needed. How can that be done without A) interrupting games to merge players together or B) block new players from joining into the group(s) that require deletion and waiting for everyone in it to die (which will be a bad experience for the last few players).
Agar.io obviously has a player population management system that works, so how do they do it?