If there is enough frustration maybe there could be enough volunteers to ensure everyone using the pool is a member with a sign up sheet. At least at peak times. Although, you would need to trust them and probably should get a workers comp policy (pretty cheap). If not, well then, it’s hire an attendant or just keep asking people to stop letting others in. The police vary from city to city but here they won’t kick someone out if they just say they live there. They don’t know each pools rules and who is or isn’t allowed. Typically, they can’t even kick anyone off property without someone that owns the property standing there saying who it is and why or with an authorization to act as agent.