Hmmm, good question. The person at the entrance gate usually has you sign a form showing that you are present and won't hold them liable. It's basically a list that everyone signs. I have never seen them (we usually park close to the gate and can see pretty well) pull out a seperate form for anyone nor have I seen vehicles turn around because they were turned away. We have also brought our son's friends from time to time and they have never asked for anything specific. Just everyones signature and $$.
No membership fee has been required of us and we've been there at least once or twice a month since June.