Errors in any contract may be corrected through amendments approved by the adoption of suitable resolutions by the contracting parties. Excluded employees may be included by groups through amendments approved in the manner prescribed for the approval of the contracts, except that if there were no members of an excluded group when the contract was entered into, an election among the employees is not required. Additional benefits for prior service provided in this part but not included in a contract, may be included through amendments so approved except that an election among employees is not required.