Group administrators in the Backend can modify other users in their group.
Go to the User section on the upper menu.
Enter the Name/email of the User you want to edit and click on his name.
Click on Edit, located on the upper right corner of the screen.
Select the group you want to modify the role on.
Check or uncheck the user roles to be granted or removed, respectively.
Click on OK
Backend group administrators are those who hold the "CUSTOMER W" and "DEVICE_MANAGER W" user roles.