I get this question a lot: Who's insured if an Association purchases General Liability Insurance?
Simple answer: The Members of the Association.
Of course, it can get more complicated. From the Chartis Sports GL, other insureds include "your "volunteer workers" only while performing duties related to the conduct of your association.
With a sports league, that goes down to the individual teams, and the volunteers working for those teams, during league/association activities. Games, trainings, fund-raisers, renting facilities, it's a big list...but here's the caveat:
"Performing duties related to the conduct of your association/league." So, MUST be members of the association.
Most Sports Orgs have some sort of player and coaches registration. I'd create one for volunteers also, just in case. Or, have each club have a list of volunteers "on file."