To invite, start you campaign (local) in the main menu, open the Co-Op wheel and select Invite Friend.
To join a friend instead of matchmaking, look to the bottom right in the main menu and select Friends (or Invites, I'm not too sure what's it called by head). There you should see a list of Uplay friends, one of which is clickable to join his world.
Note that the game relies on the Uplay friendslist, not the Steam one.