Application.Session.AddressLists.Item("All Rooms")
should do the job.
EDIT: Outlook hardcodes that list - to the best of my knowledge, Outlook looks for the address list exposed by GAL (PR_AB_PROVIDER_ID = MUIDEMSAB) with the 0x200 bit set in the PR_CONTAINER_FLAGS property. You cannot create new GAL containers programmatically, at least not from the client side.