This is purely hypothetical, just for fun. In this scenario, you don’t get to know who the GM is in advance, but they do have to run it as written. The players can leave whenever they like, but the GM has to stay until either all players leave or the players beat the dungeon.
Make sure that any “when any player first enters this room, [x] triggers” is at the veeeeeery last part of the room description, after the DM has already read the description to the players and the players have already started planning what they want to do in said room.