The More Popular Than John Lennon achievement is awarded for obtaining all 120 friends in the game. This can be quite a daunting task, especially as many friends are missable. However, it will net you a healthy 75 gamerscore.
For a full guide on all missable friends, view the Missable Friend List walkthrough.
You will collect most of the friends through regular play. Any non-missable characters you may have missed along the way can be obtained after you have completed the story, so rest assured!