Not sure why ngocphi's link didn't auto hyperlink, but per Miss Portia (Program Manager for the Xbox One Preview Program)):
"Due to a tremendous response from our fans, the Xbox Preview program has reached a near capacity level. With that in mind, we are slowing the number of new people we are accepting to optimize for testing. We are thankful for the amount of interest
we’ve received to date and look forward to sharing more details on preview opportunities soon."
So you can request an invite from this thread, but your chances of getting in at this point will be slim. Hope this answers your question.
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