I think there is still a cusotm workgroup field in 5.1 (like 4.5) so you could activate and rename that to be Admin WG then populate it for all CI's. Then you DB rule is simply to set Assignemtn To; Workgroup = CI; Admin Wg.
If you can't then maybe you could assign based on the Category of the CI? eg if Category = Windows Server then Assignmen to; WG = Server Support. But you might need a lot of these rules.