I think you have a sense of "right" that doesn't really exist. It's really a tribal thing. I prefer to use that word since terms like "association", "identification", "membership" carry baggage. They suggest some kind of formality that simply doesn't exist.

There is no endpoint because there is no real start point.

So, here's the absolute authority on all these matters: The Court of the Lord Lyon. You'll note the clans (some of them) only have a say in the name of the tartan, not in who gets to wear them.

Something to thing about: only about a hundred recognized clans, over 2000 tartans.