Players can give there user ID to a friend to use when the join Triborn. You can find your user ID by clicking your name in the top left corner then click settings.
You will be awarded when a new player joins the game and reaches the threshold for the system to award the bonus for them joining using your code.
You will also recieve awards for milestones referals such as 10, 25, 50 and so on.
Any questions on this can be sent via a ticket :)