I think that you would be able to set the permissions in the group setting... somewhere says.. (or similar)
Can create global announcement
Can respond to global announcement
Can see global announcements
Just set the group that you want to be able to reply to 1... but set create global to 0 if you don't want them to be able to make them just reply.
I have never had a need to do this, however thinking about the present CP that would be what I would look for.
Removed link due to bleedover spammers
Dawn of Iskirra