All persons, except those specifically excluded in §§ 9-1-450 and 9-1-490 and the persons permitted to exercise the option under §§ 9-1-510 to 9-1-560, who are employed by any county, municipality, or other political subdivision or agency or department of them, any eligible service organization, or any nonprofit corporation created under the provisions of Chapter 35 of Title 33, for the purpose of supplying water and sewer, after the admission of the county, municipality, or other subdivision, agency, or department of them, service organization, or any nonprofit corporation created under the provisions of Chapter 35 of Title 33, for the purpose of supplying water and sewer, into the System under the provisions of this section, shall become members of the System as a condition of their employment. All persons, except those specifically excluded in § 9-1-450 and persons permitted to exercise the option under §§ 9-1-510 to 9-1-560, who are employed by any county, municipality, or other political subdivision or agency or department of them, by any eligible service organization, or any nonprofit corporation created under the provisions of Chapter 35 of Title 33, for the purpose of supplying water and sewer, at the date of the admission of the county, municipality, or other subdivision or agency or department, or eligible service organization, or any nonprofit corporation created under the provisions of Chapter 35 of Title 33, for the purpose of supplying water and sewer, into the System under the provisions of this section become members on the date of the admission, unless within a period of six months next following the admission they have filed with the Board on a form prescribed by the Board a notice of their election not to be covered in the membership and a duly executed waiver of all present and prospective benefits which would otherwise inure to them on account of their participation in the System.