HOW DO I PLAY WITH STREETPASS FRIENdS!?

By "joining you live", do you mean on or offline?

You can only play with them if you manage to come across them online or meet them in-person.

look it up on Youtube.

You can't play with StreetPass Friends, nor are there StreetPass Friends. StreetPass is primarily a method of sharing data wirelessly with people who are using the same software; in the case of MarioKart7, it allows people to give and receive new parts for their karts (characters must be obtained by beating 150cc cups, or beating a whole class for the Mii). If you want to play with people nearby wirelessly, you use local multiplayer. Trust me, I felt like an IDIOT when someone had to point it out to me, but at least you're smart enough to ask (air-go: you're smarter than me)

Street pass friends will randomly show up in races that they have created thier own cup with, they are just ghost data however and not a live player. In order to play with them live you have to reister them as a friend either by code or via local registration.

If you use streetpass to race someone, it isn't that people; it's just the computer controlling them.

But the player's car is the same of the profile. But it doesn't affect the gameplay very much... Because the computer player wants to race close to you.

If you want to race the streetpass guys anyway, go to the mario kart channel, go to "streetpass" or something like that and select someone to race... err... controlled by the computer.

After that you can choose if you want to race on the people's favourite course (All opponents controlled by the computer), or race against an ghost of the player.
You can also join the player's game if the player is already playing online. If not, you cannot choose this.

They go to Friends List and go to you Mii and select join game.