Am I missing something? I tried to get into a multiplayer outpost with my friend, but couldn't find a way to invite pre-game lobby so we were guaranteed to be on the same team. We just had to keep trying to join the same game with fingers crossed we would both get in and be on the same team! Is there a better way I'm just not seeing?
I suppose you have already started a new game and played past the tutorial phase to enable co-op mode. And also added your friend to UpLay friend list.
1) From the main menu of FC4, select "Continue [ONLINE]" to continue your current campaign in online mode.
2) When the game has loaded, press "F1" (or F2 or F3 or F4) and select "CO-OP" from the top menu.
3) Select "Invite friends" from the left side.
4) The Uplay overlay should now open. Find your friend from the friend list and click that little downward arrow next to his/her name.
5) Select "Invite to game" (or "invite to co-op", not sure what it says). Your friend must of couse select "join" or something like that when he/she is prompted the question.
6) Enjoy
I have only played as a server so I am not 100% sure what the text says on your friends screen when you invite him/her.