The problem is, there's no way to tell an unlisted person that because we don't have access to their profile page which includes the ability to send them a message explaining this.
Another unlisted is asking to become a volunteer today. The only communication open is to thank them for asking to volunteer, which I've done. Hopefully, the person will notice and send me an email when no one confirms her. Otherwise, I don't know what to do except wait or contact an administrator who can confirm her.