(1) Each employer as defined in 39-71-117 , MCA, who has an employee in service as defined in 39-71-118 , MCA, for any length of time must be bound by the provisions of plans no. 1, 2, or 3, unless an employment is exempt under 39-71-401 , MCA. The employer, with the concurrence of the employer's workers' compensation insurer, may elect to bind the employments that are exempt under 39-71-401 (2) , MCA.