If you are going to church, but have not joined the membership, what does the Bible say about becoming a member of a church?
The Bible says nothing about joining a church. It does talk about God adding people to the church. "And the Lord added to the church daily such as should be saved.” (Acts 2:47) Just being immersed for forgiveness of sins caused one to be added to the church. Of course, that is talking about the church as a whole.
The Bible says nothing about joining a congregation of the church. Maybe that is because at the time there was apparently only one congregation in each city. If you were in the church and in that city you were automatically part of the congregation. Since that time men have added denominations, each of which has its own man-made rules for adding people to a congregation. We have also added more than one congregation in some cities. This means that we can pick and choose which congregation we want to assemble with, and which elders we want to guide us. The only guidelines for making that decision would be our own personalities and desires. The problem is that some people choose never to identify with a given congregation.
When one chooses not to identify with a particular congregation it is like living in a country without ever gaining citizenship. You take advantage of some of the benefits, but also miss out on some. You try to have the rights without the responsibility. My personal feeling is that a person who regularly attends a congregation should be considered a member. The elders have a responsibility and right to shepherd that person, and correct him if he is wrong. The deacons have a right to expect that person to help with the work of the church. Nobody should have to “join” a congregation because by attending on a regular basis they are already a member.
See also my article "Too Many Congregations" for more about the disadvantages of too many congregations.