Sorry you're having issues getting your preview invite. Not everyone is accepted into the Xbox Preview Program. If you've been selected you will receive a message from Xbox Live with instructions on how to install the app on your Xbox One.
Also you can try to access this forum section below and post questions there. If you receive group not found, then please reply to this post. A forum moderator will review your account and grant permissions appropriately if needed.
Xbox One preview forum.
MCP. MCDST. LVL 80
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