Manage costs for a college students group retreat

You're the leader of a large college group at your church and the annual retreat is getting closer on the calendar. The hassle of collecting the registration fees is looming. Your students meet every week in the midst of busy, busy lives and often can't be counted on to remember to bring cash or a check. You don't want anyone to miss out who can go, so you're looking for a solution that will combine the convenience of online payment with some kind of registration mechanism. So there's no more forgotten money and less work collecting registrations for you.