It's a new feature on Youtube (currently only invite-only). Good news is I had 25 invites. I already sent some out to some of the people here, those that have added me on YT anyway. Which is Nyubis, VG and Daxter. I think.
Anyway, I have 20 invitations left. So if you want to try it out, add me as a friend on YT (guess what my username is) and I'll send you one.