A user can be part of one or several groups. Groups are used to manage users that would have same access rights.
In groups list, clic on icon for group edition.
Using the 2 red arrows between these 2 lists, users are added or removed from a group. You can select several users at same time using right clic during selection. You can also select users through the two red arrows between the two lists, users are added or removed from the panel. You can select multiple users by right-clicking with the mouse and moving on users simultaneously. A discontinuous selection is possible by clicking on the users and at the same time pressing the button «Ctrl» (Control) on your keyboard.