If you have selected a group from the user groups list, then the third list (members list) will show all current members of the group. To add a user to the group, click the "Add" button. This will open a user selection dialog:
Figure 2.7.28. Adding group members
Select the users that shall be added to the group. You can easily select all users by checking the checkbox in the header of the list. Click the "Add" button to close the dialog and to add the selected users to the group.
To remove users from the group, select one or more users in the members list and click the "Remove" button.