the hack needed is already written, well sort of, here's how I would do it(of course this does depend on the size of the mailing list!) assuming you can create email forwarders on your server, create a forwarder for everyone in the group then create a mailing list(again assuming your server gives you nice site management tools) then just install the already made and easy to install hack 'Email Notification on reply' and use it!
Well that's how I'd do it as a non programmer, or should I shut up
