Membership was the status of belonging to a group of people or planets. Several worlds were Federation members.
As of 2369, Professor Woo had suspended Vash's membership in the Daystrom Institute Archaeological Council twice due to her having sold illegal artifacts. (DS9: "Q-Less")
The term membership was also used by the Sheliak, for example to refer to a group of colonists on one of their colony ships. (TNG: "The Ensigns of Command")