Membership means different things in different churches. In the Episcopal Church it’s very simple. To be a member you have to be baptized and you have to have your name recorded on the membership rolls of the church. That’s all! You’re in! If you haven’t been baptized, the vicar will be glad to discuss baptism with you. If you were baptized in a different denomination, the Episcopal Church accepts your baptism; baptism is once-and-for-all, we don’t rebaptize.
On another level, membership is not so simple.