How do the NPs get paid? Are they on salary, paid by an organization they work for? Or are they each paid directly by the insurance carriers?
You mention a group NPI number. You always bill with the NPI number of the legal entity that is the billing entity that expects to get paid. In this case it seems to be a legal entity that will then turn around and pay a salary to the NPs.
To clarify, just ask one of your NPs. If they are on salary, it appears that the organization is the billing entity that expects to get paid by the insurance carriers.