Hi,
Sorry for the lack of response there.
However, what you are asking for is not possible. It is a basic free RSVP system for people to say, hey I am going to this event. For that it works great.
What you are looking for is many times more complex then that and I would not even describe that as a RSVP, but a booking system.
If you think that is something you would like as a feature I encourage you to visit our User Voice Page and either upvote an existing request or make a new request there.
http://tribe.uservoice.com/
Cheers