@aidanheerdegen great news!
It appears I have mistaken, and that there is such a feature, but it is quite hidden.
Your individual users will have to access it via <site>/my/messages/group/<group-name>. So they can toggle the notification 
/ 
as and when they would like to avoid notifications for the group. Is this what you are looking for?
(Special thanks to @moin for pointing this out. 
)