What happens to players who already joined if I lower the number of slots?

If the new number still fits everyone who joined, nothing happens to them. If it is lower than the roster, the players who joined last (the most recent sign-ups) are moved to the waitlist, not removed, and each gets a notification. Whoever paid online for those spots gets the payment back to the Playbox Wallet; a cash spot has nothing to refund. The earliest sign-ups keep their places.

Example: 18 players and you set 12 slots: the 6 latest sign-ups go to the waitlist and are refunded. If a spot opens again, the waitlist works as usual.

You change the slots when you edit the game, before it starts (Gameplay, Limits). Registered players are notified about the change.