To add user(s) to a group login to the MSPControl and click on Groups,
or go to Organization > Organizations > Groups.
You will see list of all groups. Click on the name of the group you need.
Start typing user names in a Members field or select them from the drop-down menu and click Add.
Done! Users and / or mailboxes has been added to this group. If you added some accidentally, check corresponding check-boxes near their names and click Delete.
As usually, Save Changes will redirect you to the list of all groups, and Save Changes and Exit will leave you at the same page.