There are several methods you can use to get this achievement, it depends on the number of additional friends you have with you at the time. I will do my best to list several different solutions for this, hopefully one of them will better suit you.
3/4 friends in a GalleonIf you have several friends with you then the best place to get this achievement is on the
Ferry of the Damned. This will involve you and your team being dead and waiting with instrument in hand on the Ferry until another random from the same server shows up (is killed). Since this is a
"safe zone", you can't be attacked and if someone see's 4 pirates playing instruments they will likely join in! (This is how I unlocked the achievement). Continue playing until you get the achievement. These achievements are known to unlock several minutes after you complete the requirement, so be patient.
Solo/2-player SloopThis is the harder of the two solutions, this requires more luck as you need to actively find other passive players, preferably a Galleon with at least 3 other pirates. My advice would be to not hang around outposts. Although this might provide an easier meeting spot, players are typically heading there to cash-in loot and are thus more sceptical of other ships and take a considerably more offensive stance. Instead, just sail around until you see fellow ship(s), ensure you are in game chat so you can communicate your passive intentions. Use friendly gestures like the "Ahoy!" and "Aye" presets. Ensure you're weapons aren't drawn etc. With a little bit of luck you'll friend some friendly pirates. Alternatively, you can try the galleon solution above, however this may take longer as you require more people to be dead on the server at any one point.
One final tip: Check the chat options whilst holding an instrument, selecting those will make it clear to those without a mic what you want to do.
Any advice, hints and tips? Comment below