“Membership” is spoken of in Acts 2:47: “The Lord added to the church daily such as should be saved.” The church in Jerusalem began when about 3,000 were “added” by the Lord. “They continued together in the apostles’ doctrine and fellowship, in breaking of bread and prayers” (vs 41f). This act of adding Christians together was a heavenly act, not an earthly enrollment.
However there were soon many congregations of Christians in different places. A Christian may move, or places of assembly may change. By placing membership a Christian is introduced who has been previously unknown in a local congregation. Thus fellowship is established and responsibility and zeal continues. The Bible mentions several ways by which men have sought and procured this introduction.
By oral introduction. Paul came to Syria and Cilicia and was unknown in that region among the churches there, Some were afraid of him. But Barnabas declared to them about Paul’s authentic conversion and faithfulness (Gal 1:21-22; Acts 9:26-27).
By writing a letter. Col 4:10 and Romans 16:1-2 tell how a Christian can become acquainted by a letter of introduction from a trusted source. Several brethren may write a letter commending the faith of another (Acts 18:27).
By self-commendation and a request to be received. Paul did this (2 Cor 3:1-2).
If there are questions of doubt, elders may want confirmation by contacting your place of former residence. Leaders in the local church who “watch for souls” (Heb 13:17) need this information for their further shepherding.