Md. Code, Courts and Judicial Proceedings § 5-414

An association or organization participating or interested in a labor dispute, as those terms are defined under the Labor and Employment Article, or an officer or member of the association or organization, may not be held responsible or liable in a civil action at law or suit in equity, or in any criminal prosecution, for the unlawful acts of individual officers, members, or agents, except on proof by the weight of evidence and without the aid of any presumptions of law or fact, both of:

(1) The doing of the unlawful acts by persons who are officers, members, or agents of the association or organization; and

(2) Actual participation in, actual authorization of, or ratification of, the unlawful acts after actual knowledge of the unlawful acts by the association or organization.