I think my memory is going, only when I read Judith’s post about Apcala did I realise that I’d probably been asked not to invite people to Elgg, whoops!
Either way the good news is that you don’t need an invite… you can go to Apcala and register yourself (and make sure you add me as a friend :o)
Either way this is worth exploring… I don’t think it’s open source though which is a shame as I reckon there are a lot of possible K12 uses going on, more on that later.