I posted this in IdeaJam ages ago. It got quite a few votes but didn't go any further.
It would be able to restrict when - for example - only an Internal Comms team is able email an all employees group.
There's no easy way to do it in Domino world, and Exchange has had this for ages.
Note: Hiding the group from users in the Domino Directory doesn't work as address lookup use's the server's id, not the user's.