Similarly to how in a lot of good states/countries people are automatically registered to vote when they reach voting age(even though they sometimes chose not to vote) a very nice system to have would be to automatically register one for a union when they get to working age (plenty of general unions exist that accept workers of all stripes).
Or alternatively when they get a job at a workplace. Like if you get hired as a cashier you will automatically be registered as part of a national cashier union.