BuddyPress.org

Thanks everyone. BPMU sounds interesting, though I haven’t found anything about it online so far. The multi-network plugin is very cool as well, especially the idea of having several super admins, will keep an eye on that.
Pagan Gwynne: the groups solution is pretty smart, for now that’s a first step to what I want to achieve so I will test it out and post my experiences here.