One issue here is that even admins should not be able to enter certain groups. I know, this is impossible, but it clearly shows the limits of online as opposed to offline.
That sounds like that would dangerous. If something gets out of hand, who’s to put a stop to it or at least force them to calm down. And if it’s just person, would you want that person to continue causing trouble for the rest of the group?
The administrator is responsible for the entire community and therefore should have access to everything, groups included.
That is probably true if the discussion is about games, and if the users are anyone who has an opinion.
But for an invite-only Discourse among members of a political party, the more important aspect is that they can develop ideas and positions before they share those with everyone else.
This goes back to my point that online does not (yet) reflect offline. Offline, we all seek out quiet places where we can talk with another person in private. But online this does not seem to be a concept at all. Or this is only possible on Signal, Telegram, Threema.
There’s a plugin for Discourse called Discourse Encrypt (for Private Messages) that allows people to encrypt their personal messages. So, I imagine, while it may not be possible to create groups or categories where an admin doesn’t have access, I believe people can create group personal messages that are encrypted, thus preventing admins from viewing the group message’s content.
Perhaps that solution would work for you.