Really? I had no idea. So what is it I have to do exactly. I'm afraid I'm not getting this. Sorry :(
Just add the community code in your mariokart. And tell everyone to add it to theirs to. When tournament time comes hit join community(you would create a room if there are no other rooms in progress) and wait for others to join. More than one room can be created at the same time so I don't know how this would work with the tournaments. Also, if you want, you can create new VG communities with different rules,( no or few items usable, CC etc).
One thing though I think this codes should be hidden from lurkers somehow.
My 3ds friendcode: 5413-0232-9676 (G-cyber)