I did find this one. But may need some fixing at least for Tests-passed
Was having some issues getting to work right. Ie I could post as admin to show content to groupA. It will display for me Bing in that group but not my test user in that group
Blockquote from formatting bar may expose content if used. Would likely want to turn edits only viewable by staff…
As Jay said though a clever user could get around this. Plugin would be best with sensitive info.
If Self Hosted maybe a link to a Private topics(plugin) enabled category. Where only Staff can create topics and say your group can see. A link to a post in the private category should only work if current user is staff or your chosen group