We recently found out that Microsoft will be embarking on a beta program soon for its Xbox app on Windows 10. The Xbox One Smartglass app had an open beta and Microsoft is adopting the same approach here versus a closed program like the Xbox One Preview. Now, it seems as though Xbox (beta) has made a premature appearance in the Windows 10 Store.
But it's most certainly not ready for use yet.
We've been able to install it on our Windows 10 PC but on the singular occasion the app actually opened, it appeared to be an older build than the main update pushed earlier this week. Not by much, but older is older.
Mostly it refuses to open properly and sits there on the green Xbox logo screen. Since it's obviously not ready yet we haven't yet tried it on Windows 10 Mobile.
If there's nothing else we get from this, it's a Store link for when the beta program does properly go live. It isn't the first beta app Microsoft has slipped into the Store ahead of time, with the same happening with Translator 10 beta recently. In any case, the link below is the one you need.