In case you were wondering if private militias are a thing, Professor Mary B. McCord of Georgetown will set you straight in Lawfare:
Although it is widely believed that the Second Amendment protects the right to form private militias, it does no such thing. The Supreme Court made this clear in its 2008 decision in District of Columbia v. Heller, explicitly reaffirming its own 1886 holding that “the Second Amendment … does not prevent the prohibition of private paramilitary organizations.” Indeed, they are prohibited by state constitutional provisions or statutes in all 50 states.
The constitutions of 48 states include provisions that require the military to be at all times subordinate to the civil authority. That means that private, unregulated and unauthorized militias—operating wholly outside of the civilian governmental authority and public accountability—are prohibited by state law. There is good reason for this. As prominent historian and scholar A.E. Dick Howard wrote in 1974 in “Commentaries on the Constitution of Virginia,” the Virginia constitution’s ban on private militias “ensures the right of all citizens … to live free from the fear of an alien soldiery commanded by men who are not responsible to law and the political process.”
So if someone shows up at your door proclaiming some sort of authority as a private militia, tell them to get out of town and that they’re under surveillance at that very moment. Even if they’re not.
