I was searching for this same thing earlier tonight and my buddies and I gave up. We had one person on an Xbox One X, one on an Xbox Series X and one on PC. None of us could connect to each other tonight. If this is a known bug then that's good to know. This message only came up on the two Xbox's, the guy on PC didn't see this error but would time out trying to connect to either of us. Once in trying, PC guy's game crashed, but who knows if that was related.

"Generation Zero" attempting to connect to a friends multiplayer session.

Since this is such a new issue I'm going to write out the error so other people can find this, hopefully a resolution comes soon.

Invalid Game Session

Your game sesion is offline, and players are unable to join. To create a new online session, exit to the Main Menu and start a new session.

Press A for OK.

This game is really fun so far. I see a lot of people complaining it doesn't have nearly enough survival elements, but that's exactly why I like it. If I wanted a game like Rust or Ark I'd play those. If they can get crossplay multiplayer to work I feel like this is a game my friends and I would spend a bunch of time on. Would have never known about it if it hadn't come to Gamepass...