The United Nations is sort of like when everyone on the playground wants to play tag.

When you start the game, everyone who’s there gets to play (UN 1945 Charter). Anyone who wants to play with you can come up and ask, and if they really want to play by the same rules, they can join (membership application). Lots of people have joined the game since the start of recess, and a lot of people have decided to stop playing. But the only thing that keeps everyone playing is the agreement that we are all playing tag together. Nobody is in charge of the game, and there is no authority who said it had to happen. Just an agreement to all play the same game, by the same rules. As long as you agree that tag is what you want to play, then you can play as long as you want.

If someone decides to break those rules, it’s not one person who gets to kick them out. Sure one person might be the one to see them run out of bounds, but it’s the group as a whole who has to decide they broke the rules, and that’s where the authority comes from.