The Xbox Preview program lets you preview console updates before they are released to the general public.

Post your GT below.

You get access to updates earlier
You can get reverse compatibility on some games.
You get the Xbox preview program APP on your Xbox one.

You will get a message from Xbox Live stating that you have been invited to the program and will ask you to download the preview APP.

Please understand that it might take up to a week to get the message. Try to get a lot of people to invite you the more invites that you get the better Your chances are at getting into the program are. There is a slight chance that Microsoft may not select you but don't worry keep trying to get people to invite You and your chances of getting in will rise.
Very important A user contacted Microsoft about why he was not being to be invited to the preview program and this is what they had to say."Cool! Trent, I see the Invitation that came from your friend or friends, but the Preview Program is not going through because of the previous account suspension dated Sept 15 to 29, 2014. There are 2 types of category that can be part of the program. These are: Must not a Child account and no previous or existing Account Suspension from the Xbox Live Enforcement Team."
EDIT Well it looks like i was wrong if you have had an enforcement action on your account you can be invited to the program I invited my second account that had an enforcement action on it because i had a bad gamertag and they made me change it. Well 2 days later i got a message saying welcome to the preview program so maybe it is for certain enforcement actions on your account that wont let you in. Well here is some proof below.Maybe it is only for suspensions and not a gamertag change im not 100% sure.
https://gyazo.com/815e6dcda2c39060523b8e05210c1db1.png

Contact Xbox live at support.xbox.com/en-US/ (bottom of the page)
by phone 18004myxbox
or go to enforcement.xbox.com/ and it will show you all your suspension you ever had on your account.
Tell them something like my friend has invited me to the Xbox live preview dashboard and I have not received it yet and I was wondering why, thank you.
If you do have any suspensions I would recommend making a new free Xbox live account and post that one and make sure it is and adult account.
If all that fails and you still cannot get in don't worry the new dashboard will be out on November 12 2015.

Xbox 360 Games Available To the Xbox Preview Program
Main Infomation
Vote your what games you want to see become backwards compatible here.
There may be only a little time left to get into the program look at the links below for more information.
The Xbox One Preview Program Is Nearing Capacity cinemablend.com
Xbox One Preview Program is nearing capacity
Access to Xbox One Preview Program Just Got More DifficultXbox Preview program is at 'near capacity', so new invites will be cut back
Credit for finding links about Xbox preview limitations FibreOp