Membership without payment functionality

I'd like to use the Membership plugin to restrict access to sensitive documents to only approved members of a certain level. I wanted to use the plugin for its security aspect. Is there a way to do this, allowing them to register to be a member and then, ONLY on my approval and assigning them to a membership level, allow them into parts of this site? I'd like to do all this without any credit card transaction, as this site is not designed to make money. Is this possible? Or can you recommend any other plugins?