Promo codes can be issued to welcome new users - you will get a reward after entering the promo code (vouchers, stamps etc.)
You can add the promo code at registration after entering your details and before hitting "Create Account". If you have forgotten to do so, no worries, you can also enter the code if you click on Settings > Promo Codes
For now, users can only ever enter one promo code.
If you have issues with your promo code, please let us know at email@example.com.