I remember when Google Wave was hyped up to the point where people were begging others for a beta invite. When I first got access, it seemed pretty interesting but oddly enough, nothing really motivated me to use it again.
I never understood why, beta or not, anyone would release a collaboration tool without the ability to invite collaborators.
https://www.youtube.com/watch?v=v_UyVmITiYQ
I agree that the invite only really slowed it down.