You need 8 players for this.
Head to the Plunder Valley and find a tree with 16 bottles on it (it's on the NW side of the island). There is 8 spots around the tree, each player has to stand on one of the marked spots with a musical instrument in its hands. The order is this:
West + East = banjo
North + South = hurdy gurdy
Southeast + Northwest = accordion
Northeast + Southwest = drum
You have to start playing a Becalmed shanty right after the sunset, you have approx 10-30 secs window after the sun goes down. Don't stop playing, the bottles will light up if you're doing it right.