haha yeah i know that its possible the question is how^^. I want to play with other people too not only with my sister.
Person A joins an online match by going to
Online Multiplayer-> Worldwide->Race/Battle or
Communities->[community name] if you're planning on playing in a community.
Person B has two options. Both are assuming that Person B has Person A on their 3DS Friends list.
1)In-game, they go to
Online Multiplayer->Friends/Opponents and scroll to Person A's Name and presses A to join the match that they are in.
2)On the home menu, goes to their friend list and finds the name of Person A. Their Background thing should be yellow and the "join game" button at the top can be tapped.