You will need the Full Write Groups permission to perform this task.
Setting and editing hierarchy of a group
Once the appropriate group via “Groups” in the left-hand menu is chosen, open the group and go to "Edit group" from the top right “Actions” dropdown. When adding a group, there is a tab named "Hierarchy" to indicate a parent group as well as one or more child groups (a.k.a. subgroups).
Note: By setting a parent group the current group automatically becomes a subgroup.
After setting the parent group, subgroups from this "Hierarchy" tab for your group can also be added. This can be done by clicking on the "add subgroups" button.
Editing of the parent and subgroups can be done using the same steps outlined above.
Note: Group Admins of parent groups can access to all subgroup information

