RESTfully creating 2 models at once

You can just POST a contact and have the contact create also create a
group on-the-fly if necessary (i.e., if specified group doesn't
exist), which sounds like what you're doing. I don't see anything
wrong with that. You can of course also provide a RESTful
groups_controller to allow for POSTING of groups per-se, and there's
nothing wrong with that.