Thanks for the reply!
Yea, we are both on Steam, and we can both see that we're playing the game. We've played countless other games on Steam together, all which have worked flawlessly.
We've done a couple of expeditions solo now, but it's still not working. Maybe it's a bug or something.
You do need to be done with the tutorial to play CO-op. Have you tried just inviting her directly through Steam? Basically when you hit that invite button, it’s just gonna bring up the steam overlay. So you can right click on her name to invite her or to join her game directly from the friends list. See if that works.