N.H. Rev. Stat. § 100-A:22
Membership in the retirement system shall be optional for officers and employees of the employer who are in the service of the employer on the date when participation becomes effective, and any such officer or employee who elects to join the retirement system within one year thereafter shall be credited with prior service covering such periods of prior service rendered to such employer for which the employer is willing to make accrued liability contributions. If the employer is unable or unwilling to make such contributions, a member in service may petition the board of trustees for periods of prior service rendered to such employer. Upon payment by the member of the amount determined in accordance with RSA 100-A:3, VI(b) and with the approval of the board, the member shall receive credit for such prior service. Thereafter, service for such employer on account of which contributions are made by the employer and member shall also be considered as creditable service. However, in no event shall prior service purchased as creditable service under this section be used as creditable service for the purpose of eligibility for medical benefits under RSA 100-A:52, RSA 100-A:52-a, or RSA 100-A:52-b. Membership shall be compulsory for all employees entering the service of such employer after the date participation becomes effective. Municipalities and counties may, by action of their city council, board of selectmen, or board of county commissioners, exempt their chief administrative officer, at the time of initial hiring or appointment, from compulsory membership provided herein. The chief fiscal officer of the employer, and the heads of its departments, shall submit to the board of trustees such information and shall cause to be performed with respect to the employees of such employer, who are members of the retirement system, such duties as shall be prescribed by the trustees in order to carry out the provisions of this chapter.
RSA 100-A:22
1967, 134:1. 1973, 343:1. 1986, 104:1. 1989, 226:1. 1995, 269:4. 1999, 165:1. 2002, 146 : 1 . 2004, 216 : 5 . 2006, 178 : 6 , eff. July 24, 2006.