For years gospel preachers have taught "one does not join the church"; and members have parroted "one does not join the church, one is added to the church." The distinction is valid if properly understood; but I fear it is not widely understood, and often the words become mere "church of Christ" nomenclature.
Receiving Christ gives one "power to become" a child of God. (Jn. 1:12) Of "become" Robertson says, "to become what they were not before." The new baby does not "join" the status of son or daughter—it "becomes" a child, and a member of the family. Of the new life, Paul wrote, "If any man be in Christ, he is a new creature; old things are passed away; behold, all things are become new." (2 Cor. 5:17) Coming into Christ one becomes a part or member of Christ's body (Eph. 2:13,16) thus one "becomes" a member of Christ's church. (Eph. 1:22-23) Now take it slow and easy—and read the scriptures cited with great care.
One does not "become" a Christian and then, at some later date, "join" the body of Christ. One "becomes" both a Christian and a member of the church of Christ by one and the same process. The "church" of which we are writing is, of course, that one great universal body of saints; those called out of darkness by His gospel. This church is not a "church-hood" or "brother-hood of sister-churches"—if you can picture such. It is neither all the churches, nor a special portion of the churches. It is simply a figurative (or "spiritual" if you prefer) gathering or assembly of saints, of which you may "become" a part.
But when you have become a member of the body of Christ (His "church" in the universal sense) you will, if possible, "join" with other disciples in your worship and service of God. Paul sought to "join" with the disciples in Antioch—at least he was no longer considered a part of the Jerusalem company. (Acts 15:22)
These local groups, or fellowships, (such as the saints at Philippi; 1:1) were also known as "churches" (1 Cor. 1:2; Rom. 16:16) and exemplify the one and only church organization known in the New Testament. Perhaps one may speak of "joining" the local church, and do no injustice to language; but we should understand that we "join" with other disciples to form a local church. "Join" (kollaw) means to glue or cement together; then generally, to unite, to join firmly. In passive voice it signifies to join oneself to, be joined to." (W.E. Vine)
The true church of Christ on earth today is not a group of congregations bound together in some "church-hood" by name, creed, or practice. The true church of Christ is, today as always, the body of called-out people who acknowledge Christ as their head, and serve Him faithfully. If, in your community, there are true disciples of Christ "joined together" to do the Lord's bidding, they constitute a local "church of Christ." But don't be fooled by a Name. Test their preaching and practice by the Word of God. The true church of Christ welcomes such investigation. They urge you to "become" a Christian, and "join" them.