Something like:

                      h2 Community Permissions 
[h3 header] Member     Posting Access     Moderated      
h4 NON-MEMBERS link       [ ] Posting Access [ ] Moderated  

[h3 header] Member     Posting Access     Moderated     Additional Privs 
h4 MEMBERS     [x] Member [ ] Posting Access [ ] Moderated [ ] Additional Privs 

username1   [ ] Member [ ] Posting Access [ ] Moderated [ ] Additional Privs 
username2   [ ] Member [ ] Posting Access [ ] Moderated [ ] Additional Privs 
username3   [ ] Member [ ] Posting Access [ ] Moderated [ ] Additional Privs 
username4   [ ] Member [ ] Posting Access [ ] Moderated [ ] Additional Privs 

(where the link under member for non-members goes to the place where you control whether they can join or not)