To create a new usergroup, click 'manage usergroups' in your admin panel. Scroll to the bottom of the screen, where you see the 'add new usergroup' area. First you must select a name for your new usergroup. Then, check boxes according to what permissions you wish to grant to members of this new group. When done, click the button to create it. It should now appear in the list right below the normal three. You will be able to edit or delete the usergroup at any time later by revisiting the page.

After all of this, of course, you will need to assign users to the group by editing their profiles.