jonny come lately here (though the syntax is totally different from the original EarlyGo).
The Forum has a few topics on it now and so far around 120 NATS people have registered. At the moment there are around 5 new requests per day to top this up, though this may fall off once we're all in there.
As to knowing if other people have registered, the only way you will know is if they tell you, or you know their PPRuNe handle and you see them posting in the NATS Forum.
I have no idea of the real names vs PPRuNe names, apart from one or two who sent me this information in spite of being asked not to <img src="wink.gif" border="0"> None of that information is saved and requests to join are deleted once processed. The only thing retained is the list of email addresses so that if the password needs to be changed this can be sent to those who have registered.
I also note that as most of the topics in the private Forum also appear in some guise or other on the main Forum, the feared for dilution has not yet happened.