Most activism groups aren't really screening for membership.

Usually it's, "you want to join ? Cool, I'll add you."

Edit: Just read the article. They went out of their way to try to make it sound like this group was up to something other than legally show up to immigrant court and keep watch for heinous police behavior.

The memo did not provide any further details about the individual or their alleged past calls for violence and offered no specifics or evidence to explain why the FBI characterized them as “anarchist violent extremists”. The courtwatch efforts have been non-violent, and the FBI did not respond to an inquiry seeking specific examples of violence and did not answer questions about whether law enforcement had ongoing access to the private group.

This is why I don't subscribe to the Signal E2EE hype cult.

The fact that Signal doesn't hide the anonymity of its users, and forces everyone to use phone numbers is a huge red flag.

Yes. And the only person I know to have interacted with state security agencies in professional area has told me a few times that any security system based on cryptography is of no real use. Like perpetuum mobile, snake oil, and so on.

If your information is protected by cryptography, it could as well be protected by using "Aesopean language" or memorized by loyal courier or put on paper note in a secret place. You have a secret and a message, ultimately. If your secret place can be predicted, then your secret key can be stolen. If your loyal courier can be drugged\tortured\intimidated, so can be you or your addressee or your cryptography means' providers to give up the secret key or the message contents or to sabotage your tools.

"Aesopean language" is how they really do it for anything important, it's pretty naturally learned from culture (one case where spy movies and such show it right), it doesn't require niche expertise, and it does require common context that can't be fully reconstructed in most cases. The fuzziness of meaning is a feature, so is the disconnect of responsibility.

Unfortunately I'm autistic and impaired in that exact part of human communication, but honestly some of famous people whose jobs involve being enlightened black belt masters of that are autistic, so perhaps I'm just dumb.

EDIT: But it's funny that once I thought that the commonly imagined way this works is a trap for illiterate people, and technical means like cryptography are what really should be used. Perhaps, again, some sort of autistic compensation. Now I know better.