See this poster for suggestions. Each school has their own guidelines for registering student organizations. They typically have an information packet or handbook which details the process of registering and includes a registration form. There is often a minimum number of members required, and the group will often need to have a well drafted constitution. Don't be too upset if your request for official status is denied.
Many schools are just too conservative to allow a naturist group.
Some groups that have been successful in getting official recognition include the Caltech Skinny Dippers at the California Institute of Technology, the Longhorn Nudists at University of Texas, the University of Pennsylvania Naturist Student Association, and the University of Toronto Naturists. These are success stories, many more have failed. If you're not successful in getting your group registered don't stop there, continue on, but now you'll be organizing a more typical non-landed travel group. Most areas are not near public clothing-optional beaches, but even if you are, landed clubs offer many more recreational opportunities along with a safe, secure environment. Some cities have non-landed groups that rent facilities such as swimming pools.