I have tested this group of users for example those registered with @registered and it does not work
How can a user group be mentioned?
This would be a very bad idea unless the usage thereof is restricted to the admin group.
This feature is a spam generator tool. I'm sorry but we'll not add this feature in future releases.
A suggestion would be for WPForo to add an email message system to the Tools system,similar the to the Admin Note feature; with options to which user groups you want to message. Such would need a throttle or batch sending mechanism though.