There are far too many biblical passages where membership in the Church, both OT and NT is either taught explicitly or implicitly. Even the apostle Paul had to submit to the scrutiny of the disciples in Jerusalem concerning his doctrine before he was given the right-hand of fellowship and accepted as a believer.
Without church membership, the Church could not rightly administer the sacraments to new believers or those who have given a credible profession of faith.
And, without church membership, there could be no discipline by the elders for those who were errant in doctrine of life.
The laity consists of the majority of Christ's visible church, over whom Christ has appointed pastor/teachers to build them up (sanctification) through preaching the word and other instruction. A believer is one who has not only turned to Christ for the remission of sins and the imputation of His perfect righteousness in order to be reconciled to God, but also one who has turned away from the world and joined with the elect, aka: the called out ones, who reject sin and embrace holiness of life. Salvation is NOT simply an individual thing but a corporate joining of believers that share in common a belief in the one true God, in the deity of Christ; God come in the flesh Who lived on this earth witnessed by thousands, was crucified on a cross, was dead and then rose from the grave and Who now sits on the right hand of God, and by His power bringing all things to pass according to His infinite wisdom and power and Who will judge the quick and the dead at the end of the age. This corporate group is called in Scripture the BODY OF CHRIST... it is one!!
Paul puts all these truths together in one magnificent description here:
Ephesians 4:4-16 (ASV) [There is] one body, and one Spirit, even as also ye were called in one hope of your calling; one Lord, one faith, one baptism, one God and Father of all, who is over all, and through all, and in all. But unto each one of us was the grace given according to the measure of the gift of Christ. Wherefore he saith, When he ascended on high, he led captivity captive, And gave gifts unto men. (Now this, He ascended, what is it but that he also descended into the lower parts of the earth? He that descended is the same also that ascended far above all the heavens, that he might fill all things.) And he gave some [to be] apostles; and some, prophets; and some, evangelists; and some, pastors and teachers; for the perfecting of the saints, unto the work of ministering, unto the building up of the body of Christ: till we all attain unto the unity of the faith, and of the knowledge of the Son of God, unto a fullgrown man, unto the measure of the stature of the fulness of Christ: that we may be no longer children, tossed to and fro and carried about with every wind of doctrine, by the sleight of men, in craftiness, after the wiles of error; but speaking truth in love, we may grow up in all things into him, who is the head, [even] Christ; from whom all the body fitly framed and knit together through that which every joint supplieth, according to the working in [due] measure of each several part, maketh the increase of the body unto the building up of itself in love.