To create a new usergroup, go to Admin Panel -> Members -> Manage Usergroups. The bottom half of the screen lets you fill in details for a new usergroup to create. First you must type 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 standard ones. You will be able to edit or delete the usergroup at any time later by revisiting the page.

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