This is happening with me as well as others: https://www.reddit.com/r/GroundedGame/comments/1mco6ya/i_cant_play_grounded_2_multiplayer/
I can also confirm that I logged into my PC Xbox App with my partner's gamertag, launched the game, and could access multiplayer without issue. So it does not appear to be an issue with the game or app but the Xbox account being used.