@NfNitLoop This. Not to mention each of those 500 members are identified with their phone numbers, and there is no real indication (except "we promise") that Signal isn't passing along group memberships to law enforcement to begin with.
Just use email or whatever texting app you already have. Or use an actually secure alternative, i.e. one where you do not have to provide your phone number to use the service and there is not a single US-based company hosting all the communications.