Easy.
Once you add the group.
Goto Control Panel ยป Member Management via control panel.
search for the member.
Select the member.
Then select the permissions tab.
you will see Group Membership:
select or de-select the groups you want them in.
Maybe someday, somebody like Gizmo.
Will post in the faq page what a group is and how to utilize them.
It seems to be the hardest thing to understand, the architecture of groups to permissions.