Administrators & Registered users groups not accessible

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Creating a user group, call it 'testGroup' having the groupID (gID) 4, and assigning users to it I can access the members of this group like so:
$userGroup = \Concrete\Core\User\Group\Group::getByID(4);
$users = $userGroup->getGroupMembers();

So far so good, but if I'd like to have all admins or registered users, this won't work. Also these admins/regged users don't appear in the DB-table 'UserGroups'.
So my question is: How to access members of the Administrators/Registered Users group?

EDIT:

This is so embarassing!!!!
Everyone being a user is logically a registered user!!!!!
So it doesn't make ANY sense to have them in a group....

daenu