Hey @zantra,
at the moment you can only access Stadia if you have a founder's edition, a premiere edition or received a buddy pass (3 months Stadia pro for free).
The problem is also that only the Chromecast Ultra in the bundles of the founder's or premiere editions have the actual version to access Stadia. So as long as your Chromecast Ultra hasn't the below mentioned version installed, you are not able to use it for Stadia:
Therefore, even if you buy a controller and get invited via a buddy pass, you might only be able to play on Chrome or on a mobile device, if you haven't got the actual chromecast version installed. Here are the requirements again to play.
Honestly, I was in the same boat that you're in prior to release and I delayed buying my founders edition because I thought hey I don't need two chromecast ultras... but now I have three of them because I bought a third after I started playing on Stadia. If you have other screens in the house with HDMI inputs, you'll probably end up with more Chromecasts too, anyway. ![]()