Its called a membership form yes? That's a contract, were a person usually fills out and signs saying they must pay this amount, do these certain things, behave in this certain way. That is in a very basic form a contract with you and the country club.
I believe it might be illegal in the united states for anybody to have a minor sign a contract with out a guardians co-signing. That is why a lot of doctors forms require you, if your under 18, to have your parent sign as well.
If that is the case, then the kid is either, under there parents membership, or under there own membership that there parents co-signed. In doing so, the membership probably laid out a code of conduct. On it would probably be something very similar to what type of clothing is not allowed, how you should behave yourself because your actions could bring a negative impact to the country clubs image.