"You are now citizens together with Yahweh's people and members of the family of Yahweh" - Ephesians 2:19 (Good News Bible)
Although the Bible doesn't use the term membership it does point to the principle. Two aspects of membership are found. We can call these positional and participating. Don't be put off! It's quite straightforward!
When a person is born again his 'position' in Yahweh's sight changes. He was a foreigner - he is now a citizen. He was a stranger - now he's part of the family. He was in the kingdom of darkness - he is now in the kingdom of light. A change has taken place. Our new position is that we've become part of the Body of Yahshua, based solely on Yahweh's grace. This position is not a geographical thing. It's not a position on the planet. It's a spiritual thing - it's our position in a person, Yahshua.
The apostle Paul wrote, "For we were all baptised by one Spirit into one body - whether Jews or Greeks, slave or free - and we were all given the one Spirit to drink" (1 Corinthians 12:13). The Bible indicates that there are no racial, cultural, social or gender distinctions in our membership in Messiah - we are all equal members.
Being part of a family should mean being involved. We all have aunts, uncles and cousins we never see and some who only turn up at special occasions (yes, when there is booze on offer!). The local church fellowship is not meant to be like that. We are not meant to turn up just for birthdays and special occasions (feasts and special events, that is). Members of Yahshua need to be actively involved in a local church, helping out and contributing. Yahweh didn't intend His church be be isolated individuals but people who demonstrate what family SHOULD be. This involves commitment to the good of the group (and those outside it), using gifts and abilities, helping one another, doing good, meeting together for worship, and a host of other things.