What I actually want to achieve is not restricting permissions, but distinguishing a group of “category-specific managers” from normal users.
Both normal users (everyone) and these managers should be able to see/reply/create topics. But when the managers reply, I’d like it to be clear that they are responding in an “official” capacity for that category.
So the goal is:
Everyone: can post normally.
Category-specific managers: can also post, but their replies should stand out as official responses in that category.
When I try to add a specific group in the category security settings, it doesn’t let me save the changes. The group appears in the list temporarily, but after saving and going back, it disappears.
I don’t think having the group in the category permissions will help with their replies being highlighted. The security settings are really only for “who is allowed to do what”.
What you describe with replies standing out as official responses makes me think of the Discourse Category Experts plugin. I haven’t tried it myself, but I know you can define an expert group per category and highlight their replies.