Google Stadia's FAQ anwers:
Are you going to have cross-platform play?
We're committed to developing an accessible and welcoming environment for all gamers and plan on working with top-tier devs, which include those who want to enable cross-platform play.
So it is up to the developer if they want to include crossplay with there games. Hopefully Larian will answer this question soon. I do not want to have to buy two versions of the game to play with friends? (just a note you can't play on Stadia if your cable goes out. Since there is no copy of the game on your computer.)
https://support.google.com/stadia/answer/9338946?hl=en