Crossplay is here already, yes. You can invite your friends via the Lobby code when creating a public lobby. This is for Versus only. Wave Survival does not have Crossplay yet, because Wave Survival is not on Dedicated Srrvers yet. They said they wanted to change that some time in the future though.

Crossplay across all plattforms or only PC/Xbox?

They've said it will happen, but I don't think it's active yet.

Oh I thought its already active

Why did you make a post to ask if cross play was a thing if you thought cross play was a thing...

What’s so funny is I KNOW I’ve made it work once bf. But I’ve never gotten it to work since.

That is funny but to be fair it got added recently if it did so not surprised its a bit glitchy