The Pirates of the Burning Sea team has launched a Referral Program that allows you to invite friends to play the game, and then earn rewards when they go on to spend money in the free-to-play MMO. Here are the details:
"The rewards are in the form of Burning Sea Points. The number of points that you earn depends on how many friends you have referred and how much money they spend. At first you’ll earn 50 BSP per friend who buys a BSN, however as you refer more and more people the rewards will increase up to 250 BSP per referral. You can see a full chart of rewards on your referral program page. For friends who go on to join the Captain’s Club you’ll earn 500 BSP, regardless of whether they’ve purchased Notes or not.
You’ll also earn the in-game title “Best Friends Forever” when a friend starts playing, as will every friend that you refer."