Shift + F1 opens Origin In Game.
From there you can open your friend's list, right click, "Invite To Game".
To be honest we have never used that way to join each other's games. How we join each others games is by opening friend's list, right click and "Join Game".
Alternatively you could open a Chat dialog, and if their game is joinable it says "Join Game" on a button in the chat window.
If you don't use Origin in game, minimize and do it through regular Origin.
On Co-Op there is another option to Invite people in the lobby.
I seen a youtube of the new DLC coming I think on the 8th. It looks like it has an invite to party type button. Not sure if PC will get it or not because the youtube was showing off the console's DLC.
