How can I play together with my friend?
You have to open a private online lobby via Play Game>Online>Create Private Lobby. Your friend can then join you by either
– right-clicking you in your Steam friends list and then selecting “Join Game”,
– clicking on your Discord icon and clicking on “Ask to Join”,
– or entering the session ID after navigating in-game to Play Game>Online>Join Private Lobby.
Or, while being in the lobby, you can click on the + in the Discord message bar and on “Invite to play Unrailed!” to send an invite out to a channel.