However people are hearing about the site to even care about getting an invite, or know what it is, those are the same circumstances that they could happen upon the site and want to register. I don't think I've ever heard of a principle that stated invite only leads to greater adoption for a new site than open register, it's rather done to retard growth so the new infrastructure can keep up, or in this case to control spam. I believe slow growth, in absence of traction, is a perfectly valid way for many sites to grow and eventually gain traction.