A summons may be served on an unincorporated association (including a partnership) by delivering a copy of the summons and of the complaint:
(a) If the association is a general or limited partnership, to the person designated as agent for service of process in a statement filed with the Secretary of State or to a general partner or the general manager of the partnership;
(b) If the association is not a general or limited partnership, to the person designated as agent for service of process in a statement filed with the Secretary of State or to the president or other head of the association, a vice president, a secretary or assistant secretary, a treasurer or assistant treasurer, a general manager, or a person authorized by the association to receive service of process;
(c) When authorized by Section 18220 of the Corporations Code, as provided by that section.