I am a member of the 360 Preview Program, and am unable to access the 360 Preview forum in order to provide feedback. I contacted support via chat, and was asked to post a message here even though a support ticket was also created.
After registering one of my 360 consoles with the Preview program, I downloaded the update and used the system for about a week in order to provide consolidated feedback. When I first tried to access the Preview forum, I made sure that I was logged in with my gamertag, and received the expected "Group Not Found" error that the FAQ mentioned that I might encounter. Accordingly, I waited 24 hours before trying to access the forum again. I received the same error. I wasn't too concerned at that point, so I continued to use the Preview update and note any issues I noticed while still attempting to access the forum every few days. However, it has now been over a week since I first tried to access the forum and I am still waiting for access.