Thoughts:

If Hubski was invite only when I joined I probably wouldn't have stayed and waited for the invite. See: ello. By the time I got an invite my initial interest had completely waned and now it's sitting in my inbox, likely to not be opened. Like others, I don't see what problem this solves. If you want to deter spammers and craziness of large influxes you could always put a 24-hour barrier between account creation and posting, and let the users take care of this (note: not advocating this one way or the other). If a site is invite only I need to know why I want to go through the hassle of securing an invite, and following through with it. With Hubski, it is a much less tangible benefit than say a tracking forum, where I know that there's a plethora of music/movies/whatever for me to indulge in. Here, I have to hope that I still end up fitting in or carving a niche for myself and other interested people who also created their own reasons for sticking around. The community and users are great, and I love this place, but it's not as easily apparent to those that would be on the other side of the invite barrier.

On the other hand, an invite only system could generate curiosity and end up with some great people joining. If, for whatever reason, invites end up being implemented I like nowaypablo's suggestion the best (as of this comment). But I'm not sure I would invite people any more than I do now, which is infrequently. Though I have invited a few people who lurk around here. I don't know, hope that all made sense as I'm typing this while thinking about making an omelet.