Michigan Compiled Laws, Chapter 600. Revised Judicature Act of 1961 § 600.1923

Sec. 1923.  Service of process upon a partnership association or unincorporated voluntary association may be made by leaving a summons and a copy of the complaint with any officer, director, trustee, agent, or person in charge of an office or business establishment, and sending a summons and a copy of the complaint by registered mail, addressed to any office of the partnership association or unincorporated voluntary association.  If no office can be found, a summons and a copy of the complaint shall be mailed by registered mail to a member of such association other than the person with whom a summons and complaint was left.