With this being a TestFlight beta, there are a couple of things to keep in mind. The first is that the preview is limited to 10,000 individuals, so there's a good chance you won't be able to check out the app even if you register right away. At the moment, the beta is also only available in the US, UK and Canada. Likewise, should you get in, you'll likely get cycled out at some point so that Microsoft can test the app on a variety of different devices.
Due to an App Store policy, Microsoft says the beta will begin with a single game, Halo: The Master Chief Collection, and Xbox console streaming isn't included at the moment. If none of those limitations sound like a reason to skip the beta, you can find full instructions on how to join in on the company's website.
While a full launch is likely still months away, Project xCloud is slowly turning into a solid service. Compared to Google Stadia, Microsoft has been smart about not overpromising on features like 4K output and has instead worked to make the platform work as best as it can at 720p first.