It's pretty obvious what is meant by "right to join NATO". If NATO offers membership, a country can join it. If not, they can't. Simple, no? No need to bandy minor details about terminology.
A country has a right to engage in any alliance with other countries that agree to the joining. But in reality a country has a right to do anything it wants, or that it's leaders want. Russia has a "right" to invade Ukraine, probably not under international law, whatever that means.
A country can do anything, but there can be repercussions.