What is the best way to enter specialty groups with multiple providers in Address Book?

You can put the large groups in as their own entries, so that you can refer to the entire group. You can put the list of doctors in the notes section of the Address Book entry if you want the information.

Ideally, you would have entries for both the group and individual doctors. Initially, when you refer a patient, you may not know which doctor they will see in the specialty practice, but when you get the referral letter back from the specialist you want to be able to add that specific doctor to the patient's record.