A shareholders' agreement shall be executed:
(1) in the case of an existing close corporation, by each shareholder at the time of execution, regardless of whether the shareholder has voting power;
(2) in the case of an existing ordinary corporation that will adopt close corporation status under Section 21.705, by each shareholder at the time of execution, regardless of whether the shareholder has voting power; or
(3) in the case of a close corporation that is being formed under Section 21.703, by each person who is a subscriber to the corporation's shares or agrees to become a holder of the corporation's shares under the shareholders' agreement of the close corporation.