How to only display specific content to a group in a topic?

The scenario is we would like to let users see a Topic but unable to see senesitive information in specific reply or specific paragraph, until they join a group. If we put the whole article in a private catrgory, when user click on a shared link, the user will see “oops the page is not existed” and potential lose interest to the content.

I’m thinking about spliting the article into parts and put them in different category. but it would make the whole experience very broken.

Or, like marked as solved feature, add new flags to a reply and make a theme to hide the content depends on the flag. Update: I find there is a feature called “add stuff color”, would It work in this way?

Any suggestion?