N.H. Rev. Stat. Ann. § 163-A:3

The employees of the New England Interstate Planning Commission may, upon the concluding of an agreement for coverage with the state employees retirement system, be eligible for and covered by such system. Any such agreement shall provide, as nearly as may be, for the same ratio of employee contribution to total contribution as pertains for members of the state employees retirement system generally. No such agreement shall be concluded if the New England Interstate Planning Commission has already entered into arrangements for the coverage of its employees in a public employees retirement system of any other state.