California Code, Insurance Code - INS § 11656.7

Each member of an organization insured under a group policy shall be treated as a single and separate entity as respects rates, classifications and rating plans.

Two or more policies whose experience is combined for any purpose whatsoever, shall be considered group insurance and subject to the provisions of this article unless employers insured by such policies are engaged in operations having a common pay roll or where any rating plan or rating system and the rules applicable to them approved by the commissioner under the provisions of Article 2, Chapter 3, Part 3, Division 2   1, require or permit the insurance of more than one employer in a single policy.