in the table, the userlevel column default value should be set to 1 so that when a user is registered, they are automaticly set to the regular user group.
in your admin page, you could create a select list to checnge the user level, set the values of the selct list to the levels you will offer
<select name="userLevels" id="userLevels">
<option value="3">Super Admin</option>
depends on the types of user levels you will be offering though.