It's just a promo that he has with his credit card, we do this with everyone that has a promotion with X thing, and is not something that costs him anything. We always agree to divide the costs of everything that is shared. It's not just an extra ticket, is something that comes from the money of us 3 spended on those 3 tickets, he just happened to be the one with the promotion. I find it common courtesy that if someone joins an activitie, he would pay with the rest of us. And we didn't agree to pay for his ticket. We are 22 years old, we are not completely adults and not so formal with this kind of stuff. If it was other person than his brother, then he would have had to pay.