The importance of church membership seems to vary by denomination and church.  Some churches like Capital Hill Baptist restrict access to events like Bible studies to members.  Others only distinguish members and non-members by voting rights for major decisions and rarely press the issue.   Joel Beeke of Puritan Reformed Seminary has a good article about how to be a faithful church member.  9Marks addresses how to approach the topic of membership with those who aren’t willing to commit to membership.  The best three reasons cited for joining are:

• Obedience to Jesus: The church affirms your profession of faith and has the authority of baptism and the Lord’s Supper
• The good of other Christians: You become responsible for other Christians’ discipleship and they become responsible for yours
• Your own spiritual safety: The church will come after you if you wander from the faith