And they wouldn't be wrong in a vacuum, but what you're forgetting about is contract. In order for association to commence, a union is going to present a contract that spells out what happens in that case, assuming it is of concern to the union members, and the manager will be bound to what has been agreed upon. Libertarianism isn't anarchy.

I’m just playing devil’s advocate here, I don’t actually care about libertarianism because in the US it’s a label self-applied by many kooks (for example, just look up the Libertarian Party nominees for president in… any of our elections)

By US standards, in the US job market, “unions are a libertarian concept” is something that only someone unfamiliar with US labor dynamics and the local meaning of “libertarian” would say. It’s a pointless exercise of “Look how I define this word outside of meaningful context!”

> Requiring a piece of paper that the government supports in order to obtain employment does not seem very libertarian.

Do you see no difference between a contract and a contract required by the government?

Anyway, I’m going to move on. It’s been fun talking about a definition of “libertarian” that has nothing to do with the practical discussion at hand. Kudos to your definition as it applies to the term outside of the US job market.

What government requirement are you talking about? There was no mention of this in previous comments and seems to have nothing to do with the discussion. As before, a union will most likely require a contract in order to enter into an association with an employer, but that's ultimately up to the parties seeking association. Strictly speaking they could associate without one, but disputes become harder to resolve later so it is not the best idea. Indeed, under libertarianism, the government will support the contracts you enter into by helping you enforce them. That is necessary for freedom of association to be a useful concept.

"Right to work" is a union-busting concept: regardless of collective bargaining agreements an individual has the right to their employment, whether or not they pay dues to a union.

Just wanted to correct a common mistake I see made often in these discussions.

Yep! “Right to work” is a way of framing union-busting rhetoric in a way that appeals to people that consider themselves to be libertarian(ish), so I’ve intentionally used that phrasing to try to highlight that both unions and union busting can fit under “libertarian” depending on a person’s intent.