Yeah, I too am struggling to get the multiplayer to work. I have 2 Xbox One units in the house, both show Open NAT settings. After I start a party and the other player joins the party, I launch into a game. When I arrive at the planet surface I attempt to get that player to join. The only option I see which I can select with the Xbox Party menu is the "invite party to co-steam". I cannot select the "invite party to game" option. I've uninstalled and reinstalled both instances of Astroneer and they both show updated 1/1/19.
What I am doing wrong?