Anybody who is attending this should create, collect, and share Protonmail addresses for government officials or attorneys who are attending. You will probably have to teach them how to use it. Do not release any identifying information about the owners beyond a vague description of their job duties and the region of the state that they are in. You will want to find an attorney who can file or handle civil charges, someone in the police who can handle criminal investigations, a federal agent who can handle foreign counterintelligence investigations, and other community leaders who can get the word out.