Caution: If you add a user group, they will need permissions added manually to all tables and menus. They may also need permissions added to many system tables - portals (to see portals), queries, t_group_query, views etc. If any of this is done wrong, the user will not see things they should see.
To add a new user group go to Admin > Manage Users.
Click the link on the top of the page that says "Manage Security Groups".
Click the "Add New" button to add a new security group.
To edit the settings for an existing security group, click the edit icon.