User groups and their contents can be configured on a separate Groups tab.
Here you can create new groups (New Group button ), delete existing ones, and assign users to the group from your lists.
Assigning users to a certain group using Members tab is basically equivalent to adding it to the group using Groups tab , so you can use any of these methods.
On the right side of the window, in addition to the group members tab there’s Group is visible for tab. It serves basically information purpose and displays a list of the selected group of users that can see this group and users assigned to it (more about configuring visibility see “Who sees who”).