Hi Kevin,
The groupboxes are assigned by the level of access you grant a user.
Highest, high/Mid level Providers = Provider Groupbox
Nursing = Nursing Groupbox
Secretary/Audit level = Front Desk Groupbox
The billing groupbox was a holdover in the code from when we were trying to build our own PM (old Version 7 than never really made the light of day). This groupbox at this time does not have any functionality and no users can access this.
If you did send something to this groupbox by accident, you can retrieve it by going to your Outbox (click the dropdown in the top left of your messages screen to changes your boxes), selecting the message, and clicking the "Retrieve Message" option.