There is no restriction on who can hire a nurse practitioner in California. However, there needs to be a contract between a NP and a physician to supervise.
"In California, nurse practitioners may form professional corporations, but there are no restrictions on what type of entities can employ nurse practitioners. Nevertheless, the employment of nurse practitioners is generally restricted due to the physician supervision requirement."
What's interesting about this supervision, the supervision really has to do with prescriptions for drugs or medical devices to patients. Not their medical records etc on a daily basis, like you would a medical resident or PA
"If a physician employs or contracts with nurse practitioners (“NPs”) or physician assistants (“PAs”), the physician must properly supervise these mid-level practitioners to avoid liability under Section 2264. With respect to NPs, a physician: (1) may only supervise four at any one time, and (2) must develop written standardized protocols and procedures in order to allow the NP to furnish drugs or medical devices to patients. With respect to PAs, a physician must exercise control by one or more of the following: (1) examination of the patient the same day care is given to the patient by the PA; (2) countersigning and dating all medical records written by PA within 30 days; or (3) review, countersign, and date a minimum 5% sample of medical records written by the PA."