It isn't going to hurt it tipping it back, a good way to keep from doing that is to hit the rear brake when you get up to far. It just take practice until you get a feel for that balance point. No one can really tell you how to ride wheelies, just practice.

As for riding passengers the only thing I think you would hurt is yourself or your passenger if you are trying to do tricks beyond your capabilities. The quad can take if you know what your doing. I would just master your stuff first so you don't take a chance on hurting someone else. Just be safe and wear your helmet.