Top message: Secret Santa by SavageKitty
Replying to: Re: BAHAHAH! by Jiangshu
You click your friends list, and there's my card... with a picture of my Mii playing my 3DS. Up at the top, it says "OK to Join" or something like that, and then a button that you can press to automatically launch the game (if the card's in or it's downloaded to your system) and join my session (if there's an open slot... otherwise you spectate until it's free).
It not only allows online play, but is surprisingly streamlined.