The Grundy County Historical Society had 291 members in 2014. The membership roles are managed by a Membership Committee chaired by Nadene Fultz Moore. She maintains regular hours at the Heritage Center and may be reached there. The members are the base of the organization and meet annually on the second of June at 2:00 P.M. each year to elect officers, standing committee chairs and directors and conduct such other business as may come before the meeting. The members also meet in quarterly meetings on the second Saturdays of March and September and on a date set by the Board of Directors in December. Historical or cultural presentations are given at each of the meetings. The meeting in December is a Christmas luncheon.

Members receive a quarterly publication of the organization, The Pathfinder. There are two types of membership. (1) Regular membership is $30 per year. It entitles the member to receive The Pathfinder  in paper form by regular mail. (2) Electronic membership is $20.00 per year and entitles the member to receive The Pathfinder by email. Both types of membership entitle the member to participate in the affairs of the organization and to use its facilities.