The YouTube TV app for the Android TV platform was made official on October 30, and today, the app is now available for download via Google Play.
From my device list on Google Play, I can confirm that you can download it to NVIDIA SHIELD TV units, as well as the Nexus Player. You should also be able to download it to select smart TVs that run Android TV and Xbox One devices, so check on Google Play to see if you can download it now.
Go grab it!