I do not have a good answer. I have been a member of a church where one became a member by walking through the door and trusting that Jesus was your savior. I have been a member of a church where a 5 person Elder Board asked to hear your personal testimony and then asked questions about your faith to clarify things that you said. Each had merits.
... so I have no answers, except this:
One becomes a member of a local church by choosing to invest - emotionally and with your time and through "being real" - with others attending that church. Sharing their triumphs and pain and being open to sharing your life with them. Absent that ... one is merely attending a performance as a passive participant.