How do I invite a Steam friend to my party?

In order to play in the same party, both players must have launched Islands of Insight once. This will ensure you can appear on each others' in-game friend lists.

Once this has been done and you re-launch the game, press 'Tab' or click on the Tab icon at the top right to see your in-game friend list. Press the 'Join' button next to your friend's name. This will allow you to join the server your friend is in.

Now that you are on the map, simply press 'ESC' to view the menu, and press the 'Join' button here to invite your friend to your party.

Once they have accepted your invitation, you will be able to see them in your 'Party'.

Or they can send you an invitation, too.

You will be able to spot them easily when you meet them on the map, even if they go behind a wall.