I just got this achievement by playing Sequence 3 Mission 2 over and over. Load up the mission on replay and keep freeing pirates until you reach 500. The mission will reset your crew meter back to zero so don't worry about having a full crew!
Be warned: This will take a while as you only get about 20 or so each time you play the mission, what I had done was save this for almost last and then used this tactic to mop up the last crew members I needed!
You CAN do this after the main game has been completed as the achievement is cumulative between all of your play time. Good luck!