I believe that group membership have to do with permissions customised by the administrator.
The permissions are basically anything in between what an administrator can do and what a regular user can do.
Default groups (and therefore not shown) on this forum are:
There are some functions that can be added without the user being part of a group, like no PM before 5(?) posts, etc....
But if the Administrator had desired to set these permissions as part of a group, he could have created one, like, say 'Newby'