This achievement requires you to play a shanty with 4 other pirates (so 5 people playing at the same time). It's best to do this when you are playing with two or three more people, because you would need less random players to get this achievement.
Wait until gigantic skull appears on the map, this is special event when hordes of skelletons are occupying one of the forts. Usually, few big ships sail to this place and fight with skelletons and other pirates. When you see that eyes of the skull turned red, it means that someone is attacking the fort. Die (doesn't really matter how you do this, make sure you don't have any cargo on board!) and you will respawn at Ferry of the Damned. Stay close to the portal and start playing a shanty. Wait until more and more pirates appear on the ship and there is high chance that they will be curious what you are doing and will join you. Continue until 4 more people play with you, you may play on different instruments.
Don't worry if it doesn't pop immediately, it will be saved and will pop in next few minutes. Our group of four played 2-5 more minutes until one of us got the achievement, just to be super safe.
Credit to SilverBurchLive for confirming that you only need 5, not 6 people for the achievement to pop.