What happens to my bill when people join or leave?

You pay for seats, one for each person. During the free trial seats cost nothing, and inviting someone adds one. After the trial, when every seat is taken, an owner adds more under Users and licences, and each new seat is charged pro rata until the end of your billing period. If someone leaves, their seat is free for the next person you invite, so you don't pay for it twice. Seats you give back stay until the end of the billing period, and at renewal you're billed for the seats you have then.