The Society is an ecclesial body, not another catholic society among the many catholic societies of the Church of England. Membership of the Church is conferred in baptism (and initiation is completed in episcopal confirmation and first communion). It costs nothing to join the Church.
Therefore, the Society is not a membership organisation. The members are those baptised members of the Church of England who worship in parishes and institutions that receive episcopal oversight from the bishops of the Society and ministry from priests and deacons whom they have commended.
There is no membership fee. However, the Society is supported and administered by Forward in Faith. (The Director is Secretary of the Council of Bishops.) Those who are able to do so are therefore invited to join Forward in Faith and thereby contribute towards the costs of administering the Society.