I like to say that membership at Five Oaks is about trying to be a church that is at least as organized as the local little league teams. In little league you know who is on the team, people commit to practice and go to the games, and kids learn to play in their sweet spot while developing skills and learning to be good team players.
So when people ask me what membership means at Five Oaks, I really don't have any verses to quote saying something like, "Thou shalt join the local church." Membership wasn't necessary in the early church. Just showing up put you at risk socially and sometimes politically. You participated in the church to investigate Christianity or to grow in your faith and help others grow. There were no parachurch ministries or denominational options, just interconnected house churches.
Things have changed a bit and different denominations and churches have different membership traditions. At Five Oaks it's about affirming your relationship with Christ (so that you bring spiritual discernment to church decisions and you are qualified to serve in some roles that require a faith commitment) and committing to be an involved and contributing team member. You are saying, "You can count on me and I will do my part."
Every once in a while someone tells me they are against the concept of church membership. I doubt they are against what we mean when we say church membership. Unless, of course, they're also against registering their kids for the local little league team.