Once you have defined your groups you are ready to assign Users to the Group.
Click on one of the Groups in the Green / White grid.

Click the Button [ Assign Users to Group n ] where n is the Group you selected.

Note. If you do not see a particular user in the Available box nor in the Assigned box it means they are currently assigned to another group. You have to remove a user's assignment before they can be reassigned to another group.