"Confirmation is also the official vehicle by which a person joins the Episcopal Church."
In light of 1979 Prayer Book's theology of baptism, this is not accurate. Note, for instance, this sentence at the top of page 298 (emphasis added):
"Holy Baptism is full initiation by water and the Holy Spirit into Christ's Body the Church."
"Full initiation" means that nothing else is required for membership. A valid baptism is sufficient.
I note also the following statement from the Prayer Book's catechism:
"Holy Baptism is the sacrament by which God adopts us as his children and makes us members of Christ's Body, the Church ..." (p. 858, emphasis added).
It's hard to imagine a clearer statement of how one becomes a member of the Universal Church.
Moving from membership in the Universal Church to membership in the Episcopal Church, the following canon is crystal clear:
"All persons who have received the Sacrament of Holy Baptism with water in the Name of the Father, and of the Son, and of the Holy Spirit, whether in this Church or in another Christian Church, and whose Baptisms have been duly recorded in this Church, are members thereof." ~ Title I, Canon 17, Sec. 1 (a)
The Episcopal Church maintains that all persons baptized with water in the Name of the Father, and of the Son, and of the Holy Spirit are members of Christ's Body the Church. And all such persons whose baptisms are duly recorded in an Episcopal parish register are members of that one part of Christ's Body called the Episcopal Church.
So confirmation is not about membership. Instead, as the catechism puts it, "Confirmation is the rite in which we express a mature commitment to Christ, and receive strength from the Holy Spirit through prayer and the laying on of hands by a bishop" (BCP, p. 860). Or, to put it another way, confirmation is about those who were baptized at an early age making a mature public affirmation of faith and receiving strength to take on the commitment to the responsibilities of their baptism (cf. BCP, p. 412).
This is all clearly laid out in the Prayer Book and the canons. So it's surprising when Episcopalians don't seem to know it!