N.J. Stat. § 43:10-1.5
Any person employed by a county of the first class having a population of less than 800,000 which heretofore adopted the provisions of article 1 of chapter 10 of Title 43 of the Revised Statutes (C. 43:10-1, et seq.), after the effective date of this amendatory and supplementary act, shall become a member of the Public Employees' Retirement System of New Jersey as a condition of employment and shall be entitled to all the rights and benefits and subject to all obligations of other members of said system, provided that the board of chosen freeholders of such county has adopted and submitted to the Public Employees' Retirement System a resolution providing for such membership and agreeing that said county shall be subject to the same liabilities with respect to such members as all other counties participating in the Public Employees' Retirement System. Such employees shall not be eligible to be members of any pension fund maintained by said county for persons employed by the county prior to the effective date of the resolution adopted by said county.
N.J.S. § 43:10-1.5