Looks like you need to wait until next year at least.
When I was bought and connected Chromecast and Controller, I was sitting and thinking what is the Stadia. And where it hides. It's not Chromecast and it's not controller. Then, what is Stadia?
It is the Soul of gaming, mate.
Can you buy the Soul?!
if you have already a Chromecast Ultra and didn't buy one via Founder's or Premiere edition, you need to have a specific version on it, otherwise it'll not work with Stadia yet:
You also need to have a Buddy Pass to be able to join Stadia, because at the moment you can only play there with a Stadia pro subscription (Stadia Base is coming 2020). Therefore you need a Buddy Pass (of a founder), the Founder's edition or Premiere edition, otherwise you can't play Stadia yet.
I ordered this November 23, 2019. No invite code (I didn't even know there was one until I tried to turn this on for the first time today after Christmas).
What's the scoop. Where do I find an invite code? It is not in my email.
Geht mir grad genau so. Am 17.03.20 bestellt wurde auch am selben tag versendet. Das Gerät ist am 19.03 hier angekommen und ich habe bis heute keinen code. Jetzt liegt das blöde ding in der ecke da ich nix machen kann . Echt traurig.