Mont. Code Ann. § 19-3-403

(1) inmates or residents of state institutions or correctional institutions;

(10) county school superintendents who are required by 19-20-302(1)(g) and (2) to be members of the teachers' retirement system provided for in Title 19, chapter 20.

(2) persons in state institutions principally for the purpose of training but who receive compensation;

(3) independent contractors;

(4) persons who are members of any other retirement or pension system supported wholly or in volunteers; part by funds of the United States government, any state government, or political subdivision of the state and who are receiving credit in the other system for employment. It is the purpose of this subsection to prevent a person from receiving credit for the same employment in two retirement systems supported wholly or in part by public funds, except when the service qualifies and is applied for and the service credit is purchased pursuant to 19-3-503. A member of the retirement system who, because of employment by the state, is required to become a member of any other system described in this subsection is considered, solely for the purposes of making regular contributions, as permanently separated from service. Exclusion under this subsection is subject to the following exceptions:

(a) When an employer has entered into a collective bargaining agreement that includes provisions for payments or contributions by the employer in lieu of wages to a retirement or pension plan qualified by the internal revenue service for its employees, the employees remain eligible, if otherwise qualified, for membership in the retirement system.

(b) For the purpose of this subsection (4), persons receiving pensions, retirement benefits, or other payments from any source on account of employment other than as an employee are not considered, because of receipt, members of any other retirement or pension system.

(5) court commissioners, elected officials, or appointive members of any board or commission student interns, except that a student intern who serve the state or any contracting employer intermittently and who are paid on a per later becomes an active member by otherwise becoming an employee may, after becoming an active diem basis; member, affirmatively exercise the option of purchasing the service credit excluded by this subsection by applying to the board in writing to become eligible to receive service credit for the excluded service under the provisions of 19-3-505;

(6) full-time students employed at and attending the same public elementary school, high school, community college, persons who are members or of unit of the state university system, except that a person excluded from membership as a any other retirement student of a public community college or a unit pension of system supported wholly or in part by funds of the United States government, any state university system government, or political subdivision of the state and who later becomes an active member by otherwise becoming an employee may affirmatively exercise are receiving the credit option of purchasing in the service credit excluded by other system for employment. It is this the purpose of this subsection by applying to prevent a person from receiving credit for the board same employment in writing after becoming an active member and become eligible to receive service credit for the two retirement systems supported wholly or in part by public funds, except when excluded service the under service qualifies and is applied for and the provisions service credit is purchased pursuant to 19-3-503. A member of 19-3-505. the retirement system who, because of employment by the state, is required to become a member of any other system described in this subsection is considered, with regard to that employment, an inactive member of the retirement system, except that the member is not eligible for retirement or a refund of the member's accumulated contributions. Exclusion under this subsection is subject to the following exceptions:

(a) The employees of an employer who has entered into a collective bargaining agreement involving a multiemployer pension plan qualified by the internal revenue service and that requires contributions by the employer for the members of the bargaining unit remain eligible, if otherwise qualified, for membership in the retirement system.

(b) For the purpose of this subsection (6), persons receiving pensions, retirement benefits, or other payments from any source on account of employment other than as an employee are not considered, because of receipt, members of any other retirement or pension system.

(7) substitute teachers or part-time teacher's aides who may elect to join the teachers' retirement system in accordance with 19-20-302(4);

(8) court commissioners, elected officials, or appointive members of any board or commission who serve the state or any contracting employer intermittently and who are paid on a per diem basis;

(9) full-time students employed at and attending the same public elementary school, high school, community college, or unit of the state university system, except that a person excluded from membership as a student of a public community college or a unit of the state university system who later becomes an active member by otherwise becoming an employee may affirmatively exercise the option of purchasing the service credit excluded by this subsection by applying to the board in writing after becoming an active member and become eligible to receive service credit for the excluded service under the provisions of 19-3-505;

Exclusions from membership

The following persons may not become members of the retirement system: system and, except as provided in subsection (9), may not later purchase previous service under 19-3-505: