I believe the work around would be to create two ring groups (both with all your users), then under call coverage set inbound calls to go to the second ring group.
Thanks daveh. I talked to Adtran support in the interim and they directed me to this document:
What that page describes is putting each user in their own individual ring group. Then creating a sort of dummy ring group with no members but with call coverage rules directing calls to the various ring-groups. I didn't think it was particularly intuitive, but it's the recommended method and it does work.