Sure there are free membership site plugins out there but most are difficult to use, lack functionality and have little or no documentation and support. That is why I always recommend people to use the Wishlist Member plugin for creating membership websites on WordPress. This is easily the most feature rich, well designed, and well supported membership site software available for WordPress. I have seen complete newbies, with zero technical skills set up complex membership sites using Wishlist. Further to that, it supports and integrates with almost all of the popular shopping carts and email marketing systems out there.