Now you can use a generic bbcode wrapper (3) in a post’s content like this…
[wrap="group-foo-only"]
Only members of the group foo will see this content
[/wrap]
So the CSS will hide the wrapped content for anyone that’s not a member of the foo group.
Note that this isn’t an entirely secure method to do this, savvy users could use their browser tools or safe mode to see this content if they knew what to look for.
If you need a secure method to hide content, you’d need to utilize a feature like whisper posts for staff (Create a Whisper post) or develop a custom plugin.