The members send messages to the mailing list, that relate to the common interest, and these messages are then distributed to everyone on the the list. The same message - distributed to many. You can do that from within your own email program using the address book. Who is on the list is accomplished by the self-subscription feature of mailing lists.
People choose to join mailing lists because they are interested in the list's topic.
When someone joins a list, they send a command by email. The mailing list software automatically responds to these commands. The majority of functions of the mailing list are handled this way. This saves everybody an immense amount of time and energy. Let's investigate how it all works.