“Roll” membership in an earthly institution, even one called a “church,” is not joining the Body of Christ. I did not join the Body of Christ by joining an earthly “grouping.”
I was baptized by the Holy Spirit (1 Cor. 12:13; Colossians 2:11, 12) and circumcised with a circumcision not made with hands, and made a member of Christ by “the operation of God” (Col. 2:12). Joining an earthly “grouping” of believers does not enhance that standing, nor can not joining one hinder it.
Such a standing is available to any sinner who will come to Christ (not an earthly “grouping”) personally by faith in the all-sufficiency of His shed Blood for the satisfaction of God's Righteousness in the matter of payment for sins.