Access control mean allowing only certain people to access your site. This can be configured by setting permissions in the ZMI's Security tab, usually your wiki folder's. See QuickReference for more about Zwiki permissions.

Membership means allowing visitors to register themselves or become registered by an admin, to manage their password and site preferences, get reminders etc. There are some ZopeProducts which help with this. The easiest way to get this is to put your wiki inside a ZopeWiki:Plone or ZopeWiki:CMF or maybe ZopeWiki:CPS site. ZopeWiki:GroupUserFolder by itself may provide membership for non-CMF sites ? Next best is probably ZopeWiki:ExUserFolder .