The Add button increases the number of lines (groups).
The Remove button reduces the number of lines (groups).
The Set as default button makes the current group the default one. It is this group that will be initially selected when you select a group.
You can use the Group name column to edit the names of groups.
You can use the Path to group column to edit the path to users from this group. A double click on a line opens a special window where you can specify the path.