Here's a really good article by Matt Chandler on the biblical nature of church membership. What's the big deal about church membership, you may ask? Well, Chandler makes a good case from scripture that church membership exists in order to accomplish the following:
1. Encourage and enable Christians to submit to spiritual authority.
2. Receive spiritual care and encouragement from people in spiritually authoritative positions.
3. Keep each other accountable and restore fallen brothers through church disciplinary procedures.
4. Use church discipline and accountability to keep the church pure and undefiled.
5. Track growth - both physical and spiritual.
6. Creates a means for addressing physical and spiritual concerns within the church.
7. Creates an avenue for providing for and ministering to church members in need.
8. Keeps the body spiritually accountable to one another.
The list could go on quite a bit further, but that's pretty much what Chandler covers. Good stuff. If you're at a church and not a member, join up! It'll be good for you, both as a person and as a follower of Jesus.