There are 2 ways to get this.
1)Head over to a forum and hook up with a group of people.
b)Boosting And Matchmaking Forum
2) Grab a couple of friends, the more the better, and take turns hosting matches, one at a time. If you go this route, here are a few tips:
a) Start at a time when you can get European and North American players in your match.
b) The most popular game-type is Attack & Defend on Calypso Casino.
c) Player matches draw more people than ranked matches.
d) Players tend to not join empty games. Have your friends stick around as bait.
e) Keep starting matches even if you don't have 16. You need to keep the random players in your game, and they'll leave if you keep them at the lobby screen for too long.