Cal. Health & Saf. Code § 4840
Whenever two or more sanitation districts find and declare by resolution adopted by their respective district boards that it is for the interest or advantage of the districts to do so, the districts by their respective district boards may enter into an agreement for the maintenance of a centralized and joint administrative organization to care for the general administration of the affairs of each of the districts, and the construction, supervision, operation, and maintenance of the work of each of the districts, and for that purpose the districts may agree to employ the same engineers, surveyors, counsel, and other persons needed to carry out the purposes of the districts.
Such agreement may also provide for participation by said sanitation districts in the State Employees' Retirement System of the State of California and for the payment of apportionments of costs and the collection, receipt and distribution of pension payments by one district designated for the purpose and acting on behalf of all districts participating in the agreement in the same manner as provided by Sections 4841 and 4842 of this code. When the agreement so provides, the designated district shall have all the powers and perform all the duties of a public agency for the purposes of the State Employees' Retirement Law, both in respect to the joint officers and employees of the participating districts and in respect to the officers and employees separately employed by the participating districts.
Ca. Health and Saf. Code § 4840