I don't think they've said anything about it so far, but I would imagine it would be the same as Divinity, where the PC platform didn't matter, you could join through a generated code.
Oh yeah? For example, when my three friends played DOS2 on PS4, I could have joined them on my PC?
Well no, like if you bought it on Steam or GOG you'd be able to join each other, rather than being restricted to Steam or GOG players. I'll admit I'm not very familiar with Stadia so I could just be adding to confusion, but my understanding is it's a PC platform that streams the game to your machine, right? So it's still technically the PC platform?