I always thought of baptism as the beginning of church membership. I don't think of church membership as "local" church membership, but as membership of the one true bride of Christ. If you've been baptized, you're a member of the church and subject to church discipline (imho).
They're not talking about theology in that quote. They're talking about limiting legal liability. So the question is, what is the legal requirement to become a member, what is the signature to prove that Person X is a member, and where is that in scripture?