This achievement can first be earned in Sequence 3, Memory 2 - Now Hiring. You will start the memory at a tavern. If you open up your map you will see a blue flower icon just northeast of you across the square. Approach them and press to hire them. On the north side of the tavern you started in are some stairs, so walk up those and then head south toward the church in the restricted zone. Stay on the right side of the church and make sure that you have the dancers with you so that you are hidden. If you get ahead of them the guards will detect you and shoot the hostages. The two gunners guarding the hostages are on the south side of the church, so press when you see it pop up as an option over their heads, and then approach the hostages and press to recruit them and unlock the achievement.
You can also unlock this achievement in free play if you see a rescue pirate icon where they are going to be executed and have dancers nearby. However, those stop spawning at a certain point in the story, so at that point you will need to replay the memory above if you have yet to earn Siren Song.