Check the location where you can meet your buddies, e.g. Mike's Bar, for a small yellow exclamation mark icon. If there is one, head there. While there walk up to one of your buddies. One of them will say they need help with something. If they do, hit Y to speak with them. They should say something then you should have to hit Y again. Continue doing this until they give you a mission. Complete the mission and return to them to unlock this achievement.