My buddy got this game and we started a co-op Imperial campaign together, we played for 23 turns then called it quits for the day. The next day we wanted to resume where we left off but when I load the save, I no longer have an invite button via the steam friends list (since the in-game invite button doesn't work at all).

We did some troubleshooting for about an hour without success, we tried restarting steam, verifying integrity, restarting computers, nothing seemed to work.

Please help as I'd love to continue the campaign with my friend and this is one of the main reasons he even bought the game.

Hi, the Invite button on campaign mode should work, some softwares are known to prevent the steam overlay to show up. Since the Invite coop button opens the steam overlay, you would see nothing happening when clicking the button in game in this case. Try to press shift-tab while in game to see if the steam overlay comes up or not. If it does not come up, it means that one of your running software in the background is preventing the steam overlay to work properly and you'll need to figure out wich one. Hope this helps !