The only way to get into the preview program is to be invited by someone who is already part of the program. I have seen several forum users who mentioned that they were part of the program, so you could consider approaching them to inquire about how they joined. I would recommend against contacting them with the sole question for an invite (and to be fair, I am not sure if it works that way and if one has an unlimited amount of invites).

i could invite you message me your gamertag and ill add you and invite you and it usually takes 2 days for you to get accepted.

If that is possible, then would it be possible for me to tag along there if I PM you my gamertag?

Yeah sure matter of fact both of you guys just post your GTs below and ill add both of you and ill send the invites and when you get the invites acceptance in about 2 days let me know it worked!

I dropped you a PM with my gamertag. Thanks a lot