Authenticate to AD but only allow certain group

All, I've got authentication working nicely with MSCHAP, but now I'd like to only allow users that are members of a certain AD group. I would prefer to have this happen only when requests come from a specific client (wireless access point). In this case the idea is to have users only be able to get wireless access when they're in a specific AD group. How can I do this in freeradius? Thanks, Brian