Add a count down in multiplayer before the game starts. People would still be able to opt out from the game by pressing "unready" if they choose to in this time.
Nobody will be able to join the room while the countdown is in progress, avoiding unintentional "Quick Join" and immediately starting the game.
Nobody will be able to join the room while the countdown is in progress, avoiding unintentional "Quick Join" and immediately starting the game.