Members of a non-profit are most often defined in the bylaws. They might also be described in its articles. Your non–profit’s bylaws might use a word other than member such as supporter or congregants. 

Members are the people to whom the organization is ultimately accountable (i.e. they typically elect the directors and have a right to attend members’ meetings), or in the case of non-voting members still retain the right to come to members' meetings and otherwise participate in the non-profit.

Note: By members, we do not mean directors, also known as “members of the board”, we mean those individuals who become members to support the society in any number of ways. Members are those individuals who will vote at each AGM.