How To Find Full Multiplayer Lobbies

From the main menu, press "M" on your keyboard to get to the Social menu.

Navigate to the Groups tab in the Social menu and then go to the Popular Groups sub-section. Let the section load and you should see a list popular groups you can join. Join as many as you can.

From here, you simply go back to Multiplayer, and you will see a bunch of new people on the right list. Scroll through these people until you find someone that is joinable and is playing multiplayer.

You should now be able to consistently join full lobbies! Of course, the ping might not be so good and the lobbies will inevitably be killed by a hacker joining, but at least you can enjoy full games of multiplayer for a few matches!