The employees of the New England State Police Administrators Conference may, upon the concluding of an agreement with the State Employees Retirement Commission for coverage under 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 said system generally. The state employees retirement system shall not conclude a retirement agreement pursuant to subsection (h) of Article III of the compact if the New England State Police Administrators Conference has in force a retirement agreement with any other party state.