Assign auto CC using group parameters

[Harry Putnam]

How can I make any messages generated inside a specific group have a
CC addressed to a specific address?

Something along the line of (Using "G p"):
    ((to-address . "someone@somewhere.net"))

I tried the obvious thing: 
((to-address . "someone@somewhere.net")
 (cc-address . "someone-else@somewhere.net"))

No soap.

Message headers have only the to-address in place.