Is it even possible? I mean, you need to share the chat, and probably have it publicly joinable.
Keeping feds out is basically impossible. Sharing the group only in person could work a bit, but I wouldn't be surprised if they had undercover agents at any major gatherings.
Ah, TIL. Though I can't really come up with much that would scale well to the modern world where technologies are involved. But that might be only because I'm not thinking local enough, since that's what matters.
However, I'm sure there's an anarchist zine about exactly that.