If I raise the slots, do players from the waitlist join automatically?

Only if the game’s waitlist type is “Auto Join” and you accept cash. Then each new free spot goes to the next player in the queue, in queue order: he is added on a cash spot (he pays you at the pitch, nothing is charged online) and gets a notification.

With “Notify to Join”, or if you don’t accept cash, nobody is added by themselves: the waitlist gets a notification that spots are free, and the first players to tap join take them, paying the usual way.

You set the type in the game form, Gameplay step, under Limits: “Waitlist”, then “Notify to Join” or “Auto Join”. It can be changed when you edit the game.

For a group of friends to sign up together in one go (one person pays online for all), there must be enough free spots for the whole group at once; otherwise the group join is blocked and they can join one by one.