Also, if you're on a mission you can't find the other people to swap with. If you're not on a mission go to your menu, find a friend you want to switch with, and hit A on that person. That will give you a minimap location that makes it slightly less frustrating to find someone.
"I swear by my life, and my love of it, that I will never live for the sake of another man, nor ask another man to live for mine." GT/PSN: GnuX