One minor thing: "the public" is just the group "registered users" plus user 0, which is what ad_conn user_id returns for users who aren't logged in.
In 4.6 it's really person 0 but we've discussed making it user 0 (and Lars may've done so for 4.6.4) which means one can give the public write permissions on a particular forum (for instance) and allow "anonymous user" posts (once it's user rather than person 0 since forums joins with the users table to get first and last name ...)
