The Treasure Map is a menu option in the Sea Chest. It allows the pirate or crew to replay a completed Boss Battle.
To start, simply hit Tab or click on the Sea Chest in the lower right.
From the menu, press L or select the Lookout (Spyglass) Icon:
In the Lookout Menu, choose Treasure Map. Select the mission to replay. Anyone currently Crewed will receive a prompt to join the challenge. Once all crew members have responded, the mission will load.
Currently, if you are in the crew when the member starts the Treasure Map you do not have to have finished the quest to join in, but you will not get the the Sailing skill Leadership.