You cannot do this. There is no trigger that fires on becoming a member of a group that would fire an admin task.
Now if you had your own provisioning system then you would add the rules to this so that as long as your workflow was followed then you could do anything you could script. But just adding the user in the portal or running the PowerShell directly would not cause your additional changes to trigger.
That said, always interested in why when storage is not an issue, you want to reduce users mailbox sizes?
