Miss. Code § 55-19-13
Other counties may become members of the district after the establishment of the district according to Sections 55-19-3 through 55-19-7 and 55-19-11 in the same manner but without regard to the time limits provided for the original and necessary four (4) counties to become member counties.
Provided, however, for membership in the district by other than the four (4) permanent counties, the millage requirements shall be no less than the following:
Counties with total assessed valuation above $60,000,000 .................... 1/2 mill
Counties with total assessed valuation of at least $30,000,000 and no more than $60,000,000 .................... 1 mill
Counties with total assessed valuation below $30,000,000 .................... 2 mills
Whenever such county becomes a member, the clerk of the board of supervisors of that county shall so advise the Secretary of State and forward the name and address of the director for said county. A commission will be issued to such director by the Secretary of State. The director shall then proceed to take and subscribe to his oath of office and thereby become a member of the board of directors with the same rights, responsibilities and privileges enjoyed by any other director.
Miss. Code § 55-19-13