Thank you, DocJohn. I first saw a thread from the group in the general "new threads" list that I wanted to reply to, but when I tried to post I was blocked by a message that said I couldn't reply unless I joined. I looked briefly at the group posts from a few years ago that included a reminder from you similar to the one you posted here, and saw that there were some issues then that I hoped were resolved. I was feeling frustrated because my post had been blocked, and I joined without the consideration I would have given it if I hadn't seen it in "new threads". The format became confusing because the posts I had read that were old seemed to be a in a more private group setup, but then when I posted the setup seemed to have been switched to the forum. I guess the group threads didn't always appear where I first tried to reply.
When I browsed the social groups list, considering joining some after creating my account, I thought it was separate and I joined my others that way. I took my time and I regret having joined the political issues discussion group the way I did, as it became a source of anxiety and I felt more hypervigilant.
I think it's important that others know about the possibility of feeling frustrated enough by blocked posts, to join groups that are inappropriate for them via "new threads".