I see no obvious way to play Monaco with friends, but surely that must be an available option in a coop game that uses Steamworks?
It's a bit of a pain. From the main menu:
- Select Play Monaco
- Pick the mission
- Make sure "Play online" is selected, fix with P if necessary.
- Select "Custom match" with X
- Select "Create match" with X
- Make it private, confirm with SPACE or ENTER. If you create a public game, the lobby system will quickly dump people in your game and you won't be able to add friends :(
- Pick your class, confirm with SPACE or ENTER
- Select custom options with X
- Hit Invite Friends with SPACE or ENTER
- Do your inviting.
Done! From the same menu you can change the mission, if you want to.