Think about the difference between a dating couple and a married couple. The primary difference is the married couple have looked one another in the eyes and before God and said, "I Do!" They made a set of promises to one another, and by saying "I Do" they have also said "I Don't" to hundreds of other things. That's a metaphor for church membership. A church member has made a three fold commitment, and they live in the overlap of those commitments.
TO JESUS: I believe in Jesus. I am pursuing Jesus and striving for a heart centered on the gospel. I’m submitting to the Scriptures and believe sound doctrine.
TO THE CHURCH LEADERSHIP: I support the mission, vision, values of the church. I’m joyfully placing my life under the pastors of the church.
TO THE CHURCH BODY: I belong to a body, I’m connected to a group, serving the bride to help her flourish
Just like in a wedding, you marry or become a member of a church in the moment the commitment to Jesus, to a body, and to its leaders are formalized.