heres a link for more info on communities
From A poster
"The community feature allows players to create groups with other Mario Kart fans while setting up specific rules (speed cIass, items on/off, bob-oms only, etc) to make customized racing and battle environments. Community leaderboards will also show the most up-to-date information from recent races, each leaderboard resets every month which gives almost everyone a chance to make it onto the ranking list.
And if you choose, you could just throw your community code onto a forum for random people to join, or get a tournament going with friends by giving them your community code instead of having each person exchange 3DS codes with each other. In a sense this is somewhat like creating an open "clan" that the public can join that automatically gives a pre-chosen game type for everyone to play.
It's basically allowing every Mario Kart 7 player to set up 1 to 8 custom "rooms" with each having their own set of leaderboards. Then any number of people can join and play in that "room", under the rules the creator sets up, and have their performance recorded for other users of that group to see on the boards."
My 3ds friendcode: 5413-0232-9676 (G-cyber)