Possible to have multisite within a multisite running Buddypress?

So with wp 3.5 it's now possible to create a multisite within a multiside? How does one create this?

I'm thinking of using the second multisite as the place for a buddypress install (since buddypress is best installed as a main site, not on a subdomain of a multisite).

Once complete, would users of the main multisite have the same access to the second (sub) multisite and vice versa?

How might the membership plugin integrate with these two multisite?