Every person who directly or indirectly controls a person liable under Section 31300 or 31301, every partner in a firm so liable, every principal executive officer or director of a corporation so liable, every person occupying a similar status or performing similar functions, every employee of a person so liable who materially aids in the act or transaction constituting the violation, are also liable jointly and severally with and to the same extent as such person, unless the other person who is so liable had no knowledge of or reasonable grounds to believe in the existence of the facts by reason of which the liability is alleged to exist.