At this moment we cannot create promotional codes for discounts for your participants. The promotion codes that we use are for customer appeasement.
If you would like to have something similar for your participants, you can change the prices of your activities even when participants are registered. An example would be if you wanted to have the participants to have the ability of 25% off two weeks out from the activity then change it to full price the week of the activity.
Please speak with your store on how you can build the best offer for your participants and to allow for in-store communication on these price changes.