I play co-op a lot with random people, and I'm constantly asked about this. So here it is...

How to join a game without an invite:
Open the Uplay overlay (shift-F2) and go to your friends list. Select the person who's game you want to join, and click the three dots (...) by their name. There will be an option to "Join Game".
Note: You can only join peoples game if 1) Both of you have completed Dutch's Island at the beginning of the game, and 2) If their game is open (see below).

How to open your own game so others can join:
Main Menu > Options > Gameplay > Open in-game party > Select "On".
Note: You can only do this after completing Dutch's Island.

I know a lot of people might think this post is just stating the obvious, but I get asked about this at least several times per week. So hopefully posting it here might help a few people.

Also, add me for co-op (PC)