To get this achievement you must have played several of Trevor's missions. At one point Trevor will ask his friend wade to go to Los santos and investigate about a man named Michael. After several more Trevor missions are completed, there will be a mission start at Trevor's house. Go there and go through the cut scene. Wade will have returned with info on Michael. After the cut scene, the mission will begin. finish all the mission objectives. Finishing this mission will take you to Los santos. When you are able to control Trevor again select to play as Michael. You will spawn at his house, and Trevor will arrive shortly. This will Start the mission "friends reunited". Complete the mission to unlock the achievement.