With maximum respect..I have a conundrum for you: Imagine, that there is a tracker, which is open for registration for 2 years, then they change, and it become almost impossible to gain even an invite. Now imagine a tracker, where it's quite hard to gain an invite for 2 years. On the first tracker (because the opened period) there are 15K users, on the second tracker there are 3K users. Which one is rarer? Which one is harder to get in?