Coming from the official website...

Playing With Friends

This option lets you play with any of the friends you have registered on your Nintendo 3DS system. You can also choose to allow your friends' friends to play with you as well. Your friends can also choose to join you when you're playing a game by using the Friend List on the Nintendo 3DS's HOME menu. Note that they can not immediately join in a game that's in progress. Instead they will be taken to the lobby to wait for the next game to begin.

Looks like we've got another one like Mario Kart 7 on our hands. Thanks to Greg for the heads up!