The agency shall be composed of four members appointed by the county board of supervisors and two members appointed by the city council of each incorporated city in that portion of the county described in Section 67950, unless there is only one incorporated city, in which case that city shall have three appointed members. The appointing authority, for each regular member it appoints, may appoint an alternate member to serve in place of the regular member when the regular member is absent or disqualified from participating in a meeting of the agency.