Ubiquitin, if you are an able-bodied male US citizen between 17 and 45 you are a member of the US militia, whether you like it or not. That is the law.
But the right to keep and bear arms is not restricted to militia members. It’s not “the right of the militia”, it’s “the right of the people“. That means everyone. The same people who have the freedom of speech and the press, the right to the free exercise of religion, the right to be free from unreasonable searches and to due process, etc. It’s a fundamental human right that the Bill of Rights did not create but merely recognizes and protects.