Entrant.org and Entrants.org -- Entrant and Entrants
For Sale at Sedo:
However, Entrant and Entrants are generic words; consult with a trademark attorney or expert for advice on how this term may be used in commerce without infringing.
In addition, these domains would be perfect for any kind of contest promotion, not just charity sporting events.
One word, easy to remember, and passes the radio test.