Quickly add Users to a newly created User Group by multi-selecting Users from the list and then drag-and-drop them into the new Group name

For users who have not logged into the system for a while, instead of deleting them from your account completely, simply deactivate their account.  Deactivating the user will put their account on hold and allow you to again activate them once they need access to the system. All of the users file and folder permissions will stay intact as well as any existing inclusion to 'Groups' made within the account. 

To deactivate a user, 'right-click' on the user and then click 'Deactivate'.