Cal. Ins. Code § 699

Except as specifically permitted by this code, a certificate of authority shall not be issued to an unincorporated insurer. This section shall not be applicable to an unincorporated insurer now holding a certificate of authority to transact any class of insurance in California, nor shall it affect the right of any such unincorporated insurer to hereafter apply for or be issued a renewal certificate of authority or an amended certificate of authority to transact additional classes of insurance; provided, that any such unincorporated insurer shall continue to comply insofar as applicable with the same requirements as are now or hereafter applicable to corporate insurers; nor shall this section prohibit the issuance of a certificate of exemption to the trustees of a fund established by one employer, or by two or more employers in the same industry, or by one or more labor unions, or by one or more employers and one or more labor unions, to insure employees of the employers or members of the unions for the benefit of persons other than the employers or the unions.