[2019] How can I coop with a steam friend?

Hey both. Once you reach the campaign quest The Prayers, your party tab will get unlocked. Clicking the tab will unlock the party menu - just remember to tick the "Looking for Group" box and you will see other players who are also only and looking for a party. Right-click that players' profile and you will see the option to add them as a friend ;)

The campaign is not accessible in CO-OP yet - we saw enough feedback about it that we know it's something we need to add, so it will be eventually. It's just going to take a little while to add it as we have to work out first-play progression (as you've probably seen, the campaign has a lot of cutscenes and the first hour or so of the game progressively adds all of the tabs available to you) so we need to account for this in our rework before we can make it available. It will be added eventually though :)