when you get enough girls with minimum level, you use them to give experience to the girls.
You raise the experience with books to 250, 300, 350, 400, ... (multiples of 50), then to pass to the next 50, you need to spend gems to start the next block of experience.
When you join a club, you benefit of the bonuses in attack, money, etc that members contribute to. That's also a good place to ask questions on how to play the game.
Most club requires invitation, I keep it public so members can join and try, but there is minimum level of 150 right now to join.
