I don't see why you need new features to do this. You just have a special "waiting for players" mode at the start of the game, and once it's started, reject any new people joining (or just set the room size, so the signalling server sees that it's full, and then you can also de-list it from the list of current games so nobody even tries to join it). As with any multiplayer game the host is authoritative, so the host makes the decisions about when people can join and when to start the game.