Md. Code, Corporations and Associations § 4–201

(a) A corporation may elect to be a close corporation under this title by including in its charter a statement that it is a close corporation.

(b) The statement that a corporation is a close corporation shall be:

(1) Contained in the articles of incorporation originally filed with the Department; or

(2) Added to the charter by an amendment which is approved:

(i) Under the provisions of § 2-603 of this article, if at the time of the adoption of the amendment no stock of the corporation is either outstanding or subscribed for; or

(ii) By the affirmative vote of every stockholder and every subscriber for stock of the corporation.