I just noticed this on mariokart7.com:
This game is getting better by the minute.
Didn't know Nintendo was going to make their own communities. And this seems to confirm you can actually "explore" the various communities in-game and be able to join even without the need of the code.Worldwide online* Communities, appearing for the first time in Mario Kart 7, bring players together like never before. Join a Community to play whenever you like with groups without having to add them to your friend list - or create your own Community and and have racers play by your rules.
Right from the get-go, you'll be able to join Nintendo-created Communities with their own unique rules- such as speed class or items available. Communities come in all sizesfrom two players up to 100,000, and every Community has a leaderboard, giving the best players bragging rights.
This game is getting better by the minute.