Wulf's comment to the previous post on baptism is spot on. What matters is what happens after baptism - the months and years and lifetime that follows it.
And his likening it to marriage in some way helps to answer Graham's point. I agree that 'membership' is an extra-biblical idea. But relationship isn't. And it seems to me that Baptism in the New Testament always has an eye on our relationships with one another as well as with Christ. We are baptised into the body of Christ - always understood as a local community of followers of Jesus.
Membership - if it has any value at all (and I share Graham's scepticism about it as a thing in its own right) has value because it is our way of ordering those relationships.
In the New Testament churches functioned as households or as meal time symposia and thus operated according to a set of unspoken relational rules. People belonged to something and felt as though they belonged. And the NT adds to that advice on how to ensure those relationships function well.
So Philippians 2:1-18 tells us how we need to live together in a hostile environment in a way that will ensure mutual support and accountability and corporate, communal witness to the wider world. Or Ephesians 4:11ff talks about relating well within the household of God (outlined in chapter 2).
For me membership needs to function as our contemporary equivalent of those ordered relationships. if it doesn't we should scrap it. Membership is about us saying I want to be accountable to and supported by this group of people, I want to walk with them on my journey of discipleship, I desire their companionship and insight.
For this reason, I think membership should be something we opt into rather than get invited into (as in, 'do you want to join my club'); something that is focused on me agreeing to a way of life that enables me to walk with the brothers and sisters that I've met at this particular church. In other words, membership should be a covenant that we freely and regularly enter into.
I'm not sure that answers Graham's question. I'd be interested to hear how his church and others do this.
Marcus' comment about our rules flying in the face of what God is doing by calling people to be a part of our community is also spot on. He uses examples of Calvin, the wesleys and Jim Packer. I've used the example of John Stott. none of these people can join my church - or Marcus' by the sound of it - because they haven't got wet enough. I think it indicates a weakness of our position.