Join Your Friends
Its extremely fussy. But this is how it worked for me:
Have them on your Steam friends list. Make sure they’re not set to ‘invisible’ on Steam. Then when they log in have them find your name on the in game friends list, and hit the yellow button to join your game. (Or the reverse if its you going over).
The other button, the invite one, doesn’t seem to work properly near as I can tell.
And it sometimes takes a few minutes for the yellow join button to show up on the friends list. Like I said, its fussy.