This one's pretty straight forward, and there's a few different ways to go about it:
1) Be invited into someone's game.
2) Go to the Game Lobby and search for a game to quickly jump into.
3) As you're playing, the occasional message will pop up saying that "GT X" is nearby and can be joined by pressing Left on the D-pad.
All of these methods will net you the achievement as soon as you successfully join a game.