Bringing an existing business you've been working on is against the rules. That should have been enforced by whoever was facilitating.

If the rules are well explained and enforced, and you keep an open mind, startup weekend can be pretty cool. If your idea is bad, it will get eliminated right away, and you have to be ready for that. It's also best to team up with people you don't know, and you might meet one or two people that you'll end up becoming friends with.