Go to online mode (1 or 2 players both work), tournaments, search by code, enter this: 0268-4142-9420
The thing wildfire said about joining friends is important. You can do that to start with if you're friends with any of us, but it also lists people you've recently raced. If at any point you need to leave and come back, rejoin through the friends page. If you try to rejoin through the tournament page, you'll likely create a very lonely second room. I think the game interprets your leaving as an indication that you don't want to be with that group of people.
You can join anytime from 9PM to 10PM central, but it's best to join at the top of the hour for accurate rankings.