What you would do is put a hard linked entry on your index (using entry_id parameter) and then restrict access to all other templates to only the proper member groups. Well, not all other templates - you’d leave have to make sure tha the registration and login forms remained available, of course. Then just include a link for those members in your index. You could even include the login form on the front page for those people that are registered, or do some conditional redirecting if they’re logged in, etc.