The contract shall include in this system all firefighters, police officers, county peace officers, local sheriffs, and other employees of the contracting agency, except as exclusions in addition to the exclusions applicable to state employees may be agreed to by the agency and the board. The contract shall not provide for the exclusion of some, but not all, firefighters, police officers, county peace officers, or local sheriffs. The exclusions of employees, other than firefighters, police officers, county peace officers, or local sheriffs, shall be based on groups of employees such as departments or duties, and not on individual employees. The exclusions of groups may be made by amendments to contracts, with respect to future entrants into the group. The board may disapprove the exclusion of a group, if in its opinion the exclusion adversely affects the interest of this system. Membership in this system is compulsory for all employees included under a contract. This section shall not be construed to supersede Sections 20303 and 20305.