I agree Discourse should be for everything publicly available. I think that's best for the community and also do access permissions not mix well with Google Apps (we'd have to keep it in sync with Google Groups, which is just much easier to manage.)
That also means migrating Hackership off of it, at some point, and removing the Foundation group (which is not in sync as we speak.) For private spaces — confidential information about infrastructure, closed legal details about the Foundation, and regional organizers — we should maintain our Google Groups mailing lists.
This would also make much clearer our privacy implications. I remember we had problems in the past with members being unsure about which forum is a "safe place to communicate." Such a move would make it inexplicably clear: Discourse is all open, Groups are private.
An alternative would be abandoning Google Groups altogether and only using Discourse groups. I'm tentatively meh on this one. @ben?