This subsection applies only to: an elected or appointed official; an employee of the Governor’s office; and a desk officer or employee of either house of the General Assembly who receives an annual salary as compensation for employment. Membership in the Employees’ Retirement System is optional for an individual who was elected or appointed to a position described in paragraph (1) of this subsection from July 1, 1957, to December 31, 1979, both inclusive. During an individual’s term, the individual may become a member of the Employees’ Retirement System under this subsection by completing and submitting an application for membership to the Board of Trustees. Membership in the Employees’ Retirement System is optional for the employees of a participating governmental unit who are employed by the participating governmental unit on the effective date of participation in the State systems.