The point of those groups is that they are closed. Similar to how "Secret" Facebook groups became the late 00s equivalent of invite-only PHPBB forums (think what.cd). If you're part of whatever political or ideological movement is en vogue, you're free to candidly speak on whatever public platform you choose. If you aren't... You end up inviting a few like-minded friends to a Signal group to share your real thoughts and ask questions. They, in turn, invite a few more people. After a few months, you find yourself in a full-blown community; one where you can get all kinds of useful information.
But those communities aren't interested in advertising their existence.
But those communities aren't interested in advertising their existence.