735 ILCS 5/2-205.1: Service on voluntary unincorporated associations.

A voluntary unincorporated association sued in its own name may be served by leaving a copy of the process with any officer of the association personally or by leaving a copy of the process at the office of the association with an agent of the association. A voluntary unincorporated association sued in its own name may also be notified by publication and mail in like manner and with like effect as individuals.