Wouldn't that slow it to only personal friends of people getting in? Not necessarily, I may have a discussion on another forum with someone I don't actually know and think to myself, "this person would be a great fit at Hubski," and then I'd mention it to them. -That's just me though.
However, what's wrong with people inviting people they know to Hubski? Also, with a day of "open enrollment" each month, it will allow others the ability to join. What we would not have are the HUGE influxes of new users that can occur when we are mentioned on other sites. That's not necessarily good or bad.
I'm not 100% sold on the idea, but it's worth considering.