But is church membership Biblical?
Matt Chandler (a short article here) argues that the New Testament seems to suggest some known local body of members were present even in the 1st century. Here a few of Chandler's observations and few I've added: Who was in an individual church was known (Rom. 16). People had identified leaders who had been selected by a known Christian community (1 Tim. 5:17; Heb. 13:7). These leaders had specific responsibilities over a specific group under their care (1 Peter 5:1-5). People were removed from the fellowship of a local body through church discipline by a known church body (Mt. 18; 1 Cor. 5). Local congregations voted for leaders (Acts 6), sent out missionaries (Acts 13), and made pronouncements on theology and ministry practice (Acts 15). All of these actions seem to suggest that a recognized body had been established to make the decisions. They may not have had "membership classes" per se, but there certainly were procedures set up to make sure someone did not become a member who did not hold to certain Christian beliefs and were willing to submit to certain Christian practices.
Another article I read this week (http://www.batteredsheep.com/biblical_church_membership.html) argues strongly that church membership is unbiblical. The author, James Sorrells, makes a strong case that membership seems to be an added concept to the Scriptures. All believers are a part of Christ's church, so why would there ever be an added step of a formal membership process? Sorrells point is taken and in a perfect world this might work. And maybe this idea did work in the early church when the church was quite small comparably, and when the church had a purer system of apostolic doctrine and leadership in place. But I don't think this is possible for the 21st century (nor the expectation from sacred Scripture), and I'd like to suggest some practical reasons for church membership.
1) Saving faith should be investigated: On a number of occasions in Acts the apostles were following up on people who thought they were "disciples" (Acts 19:1), only to find out they had believed in something that was not sufficient for salvation (Acts 19:2-7). Thus, one important role shepherds often do in a local church is help people make sure their assurance of salvation is in Jesus Christ alone. A membership process works both, Lord willing, to finish the work of evangelism and to start on the work of formal discipleship. To skip a membership process could end up giving people false hope, thinking that being in the church makes them a Christian. Another practical note on this is regard to the 21st century phenomenon of mobility. People move every 7 years or so. Unlike a 1st century world where most people never traveled more than 50 miles in a lifetime, we have a world of coming and going. Local churches need to have systems to screen potential members to make sure they know the Lord (and aren't just assuming salvation because they "attended 1st Church back home").
2) Doctrinal agreement is necessary: In a world where even the smallest towns have 3 or 4 "Christian" churches, it is important to make sure all the people in a local assembly are in agreement on doctrine. Churches that fail to make sure those who are in their flock are in agreement will end up (re)fighting theological battles that have gone on before. Also, when my church meets to make decisions, I want to know that those voting have agreed on the major issues of the faith.
3) Agreement on church governance is necessary: Whether a church believes in a an episcopate, a presbytery, or a local plurality of male elders, makes a difference in how vision is set and discipline is conducted. Membership allows and invites Christians of differing perspectives to find a church home that they willingly submit to the leadership God has put in place.
4) The church needs to be wise in a dangerous world: One of the most demanding practical reasons for church membership today is for protection. It is difficult to conduct church discipline without subsequent legal action unless an individual has verbally and/or in writing submitted to a church's position on theology, Christian practice, governance, etc. (and even then you cannot be protected 100%). Some might think that this is the church being conformed by the world, but it comes off to me as wisdom, pure and simple. As long as we do not have someone sign something that isn't already in the Scriptures, I am not adding one bit to the Bible. Rather, I am asking people to uphold and submit to Scripture.
All this to say, the local body is strengthened and the Gospel advances when believers covenant together to fulfill the Great Commission. I believe membership is both biblical and practical, and I hope you are blessed to be a member of a local church (knowing full well that there is no perfect church; we are all just a bunch of recovering sinners, longing for the return of our King).
* John MacArthur's church has a nice write up on church membership here: http://www.gty.org/media/pdf/Church_Membership.pdf
* Tim Keller's church also had a useful explanation for a formal membership process: http://download.redeemer.com/pdf/membership/biblical_basis_for_membership.pdf
*Here is another longer and a bit more feisty article that argues not only that membership is Biblical, but that it is not optional (http://www.reformed.com/publications/ischurchmembershipoptional.php). I don't go everywhere this guy is going, but the article made me think.