Just like Rock Band 1's achievement, you need to get 1 million fans. The amount of fans you get increases with the amount of stars you get on a song. You must play on Hard or higher. The difficulty is tied down to the lowest person, so if everyone is expert, and one person is easy, you will only get to the limit of fans on easy. An easy way to do this is go to Make a Setlist in a venue, and pick any of the rapping songs on Expert Vocals. Again, use the same technique to "sing" these and you will most likely get 5 stars. Also, if you play charity events you get more fans but no money. Look at which staff member you have hired, some will get you more fans, others will get you more money.