Hey there, I have been asked by a member of MS Support to post in here to keep track of the issue.
I have spoken to Nate U today, after receiving over 20 verified invites to the preview program. I have screenshots showing me getting invited and have also invited a test gamertag which was accepted straight away. We are currently escalating this but as instructed I'm posting it here.
For the record, the reason I want in is because I constantly suffer from software issues and actually have been involved in betas all the way back from Windows Vista and The NXE for 360.