The Group Screen is where you create and edit groups (assuming you have permission).
To get to this screen: click on the Admin Link in the Sidebar to get to the Administration Screen, then click on 'Groups' at the top of the screen to get to the Groups Screen and either click 'New Group' or click the Edit Group icon for any group.
Below the form is a list of users that belong to this group (if any). To remove a user from the group, click on the Delete User Icon.
To add a user to this group, click on the Add User Button to show the list of available users. Then click on the users you want to add to the group to add them. They will be removed from the Available Users list and added to the Selected Users list.
When you have finished making your changes, press the Save Button in the toolbar on the form.