The ability to maintain a group based on org unit or another attribute is a very frequently requested feature missing from the Google Apps platform.
The best you could hope for at the moment is a scheduled job that dumps the users in an org unit, then dumps the users in the group, then figures out the delta, and adds or removes users as needed.
It would be great if you could disable users unsubscribing from a group or changing their email preference to 'No email' so communications through the Group could be relied upon.
Cliff Wakefield
Technical Lead, GoogleApps@RMIT
Information Technology Service
RMIT University
Postal: GPO Box 2476, Melbourne, Victoria, Australia 3001
Location: Building 158, 300 Queen Street, Melbourne, Australia 3000