There are 3 methods that can help you with this: #1 Boosting: Create or join a boosting session on the site for this achievement. Invite or join the boosters. #2 Adding Random People: Check the player list in a match and add random people. If they go to the Backyard Battleground, join their game. Or invite your new friend. #3 Checking your Friends List: I checked my friends and found someone playing this! I joined their game. Or you can invite them! It is recommended you add everyone that your gonna try and get this with. Good luck!
I recommend joining a friend instead of inviting them, because you can pop the "Who Can You Trust" achievement too!