The problem is that you want to hide the existence of a group from the general public.
To do so correctly, when you type @group it should not show up with any special styling for users who are not supposed to see it.
For example, the group @ninjas is not visible to the general public on meta, but here it has the special styling. So, in effect, users know that the group @ninjas exists.
To support a squeeky clean implementation here we would need for the “special” @ninjas styling only to show up for people who are allowed to see the group.
I can add @ninjas to the suggestive search, but then I also need to remove the styling to complete the fix here. Removing the styling would cause confusion if you use the feature, so its back to square 1. Cause your end users will see when they click @ninjas:
Which
- looks bad
- exposes the fact the group exists