I just came across this issue while setting up groups that can be mentioned and it then ‘explodes’ to mention all the people in the group. It is not indicated which are not going to be notified, both while posting for the author of the post and later by others reading it who might think the person has been notified and will be joining the discussion.
Unless this is such low priority that we have to learn to live with it - which is fine - this seems to be the best solution: