Hello!
Thank you for the response. So whenever the host loads the game, they send an invite to the two of us who are not hosting. Regardless of how we try to join (either direct connect code, searching on the multiplayer lobbies in the Baldur Gate itself, Steam invites from the host), it results in the error above. Whenever the host clicks on session, it gives them the ability to invite up to 4 players as intended, however only one of us can join them. We just re-tested your suggestion to confirm, and the host sending invites from the session menu still does not increase the party size or get around the error.
Thank you!
