Create a code to give a friend free access, or to run a promotion like "20% off with this code".
Pick something short, but not obvious to guess. For example, "redgiraffe", "Peter10Joy", "FBWinners88".
Generate random code
If you give the discount a name, this is visible to people who use your code, so use something friendly, like "Xmas sale", "Free access for reviewers", or "Facebook discount". Or, just leave it blank.
There is no charge for setting up this code. Your commission is calculated on the reduced price the customer pays.