I recently launched a startup that came out of a hackathon. It was and idea I had for for several months and used the hackathon to get it started. The team meshed well over the weekend but I could see the holes if we stuck together long term. Also, they wanted to do it as a side project whereas I was going to do it full-time.
My advise would be to let everyone think about what they want to do with it and then have a meeting a week or two after the hackathon. Everyone needs to be honest on how much they can contribute, what skills they bring to the team, and how serious/passionate they are about the idea. You'll probably find out that most were really excited that weekend and couple days following but then the excitement begins to fade away.