I have it set up like this before, I've done the tests with a group of 10 people and they do not receive notification, I do not know if someone else happens to thisYou need to ensure group mentions are enabled for users, and the group is mentionable, and finally the users are permitted to mention enough users to cover a group they are mentioning (ie XF mention limit, a group with 100 people but the user is only permitted to mention 10 people; only 10 people get mentioned when mentioning that group)