Not yet, unfortunately.
We are actively working on portals and authentication to provide this experience, but it’s still in progress.
Our best recommendation, although this does not apply to all private cases, is to make that Feed both SEO-unlisted and unavailable on the site’s left-hand navigation.
However, the messages will still show up on the Home Feed so this does not work for everything.
Overall, I think you will need our private feeds capability when it’s complete.