Membership in the Vermont State Retirement System shall be optional with employees who are in the service of the employer on the date when participation becomes effective pursuant to subsection 482(a) or subsection 483(a) of this title and any such employee shall become a member as of that date or as of the date of completing three years of continuous service for the employer, if later, unless he or she files with the Retirement Board within 30 days of his or her eligibility a notice of his or her election not to be included in the membership. Any such employee who becomes a member of the Vermont State Retirement System within one year of the effective date of participation of his or her employer shall be credited with creditable service covering such periods of service prior to such effective date with such employer or predecessor employer for which the employer is willing to make accrued liability contributions. Thereafter service for such employer on account of which contributions are made by the employer and member shall also be considered as creditable service. (Added 1971, No. 231 (Adj. Sess.), § 4.)