The following persons are participants in the System:

(a) Members of the Service serving under a career appointment or as a career candidate under section 306 of the Act (1) in the Senior Foreign Service, or (2) assigned to a salary class in the Foreign Service Schedule;

(b) Any person not otherwise entitled to be a participant who has served as chief of mission or an ambassador at large for an aggregate period of 20 years or more, exclusive of extra service credit for service at unhealthful posts, and who has paid into the Fund a special contribution for each year of service;

(c) Any individual who was appointed as a Binational Center Grantee and who completed, prior to February 15, 1981, at least 5 years of satisfactory service as a grantee, as determined by the Director of Personnel of USICA, or under any other appointment under the Foreign Service Act of 1946, as amended, who has paid into the Fund a special contribution for such service.

(d) Any person converted to the competitive service pursuant to section 2104 of the Act who elects to participate in the System pursuant to section 2106(b)(1) or (2) shall remain a participant so long as he/she is employed in an agency which is authorized to utilize the Foreign Service personnel system.