One thing that hasnt been mentioned on the 'invite only' front is that if you do run an program like that..
Once one of the sheep in the 95% find out that they HAVENT bee invited abou tit they will inevitably start drama about it
That in itself can be a great tool to an affiliate program, if marketed in the right way....
I would rather have 100% of 95% contacting me WANTING to join than not, at least this way i can ask them about their traffic sources etc etc