Been thinking about this all day, off and on.
Remember, one is joined to the church by Baptism; one who is baptized IS a member of the church. (Can 204) Even one not in full communion is still part of the church if they have been baptized. Those who are united to the visible part of the church are obligated further. (Can. 205)
Chatecumens are not baptized; even as they attend, they are not part of the church, yet, but are joined to the church. (Can. 206)
Can. 204 §1. The Christian faithful are those who, inasmuch as they have been incorporated in Christ through baptism, have been constituted as the people of God. For this reason, made sharers in their own way in Christ’s priestly, prophetic, and royal function, they are called to exercise the mission which God has entrusted to the Church to fulfill in the world, in accord with the condition proper to each.
§2. This Church, constituted and organized in this world as a society, subsists in the Catholic Church governed by the successor of Peter and the bishops in communion with him.
Can. 205 Those baptized are fully in the communion of the Catholic Church on this earth who are joined with Christ in its visible structure by the bonds of the profession of faith, the sacraments, and ecclesiastical governance.
Can. 206 §1. Catechumens, that is, those who ask by explicit choice under the influence of the Holy Spirit to be incorporated into the Church, are joined to it in a special way. By this same desire, just as by the life of faith, hope, and charity which they lead, they are united with the Church which already cherishes them as its own.
§2. The Church has a special care for catechumens; while it invites them to lead a life of the gospel and introduces them to the celebration of sacred rites, it already grants them various prerogatives which are proper to Christians.