Membership at the Y means you pay your dues and get your card. You never have to show up to exercise or swim.
Membership at the local Baptist congregation means you got baptized or brought a letter as evidence that you were baptized already. Walk down front and ask to join. The Presbyterians vote on whether to let you be a member or not. If you never show up for any of the meetings they might vote you out again. The Methodists made me a member when I was 12 because I took a course and passed a quiz. Romans baptize babies and make them members.
When Paul was writing the believers in Corinth about the fact that one Spirit gives each of us a gift, and the gifts are different, even though the Spirit gives them all, he told them this in chapter 12:26-27 (1 Corinthians): So if one member suffers, all the members suffer with it; if one member is honored, all the members rejoice with it.
Now you are the body of Christ, and individual members of it.
(from Holman Christian Standard Bible ®©)
Membership in his mind was clearly something more fundamental and life-sustaining than membership at the YMCA. Literal membership means being a part of something bigger in a genuine way. Dismemberment in medical terms would mean that your finger or leg was cut off your body. It would make a big difference to no longer be a member in that sense.
That is how we are members of the body of Jesus. We don’t join. We are grafted into Him as branches in a life-giving vine. And we are not just connected to Him, but to each other – with needs and responsibilities. When we acknowledge both of those requirements, we are more useful body parts, and as a result more useful citizens of the Kingdom and our neighborhoods.