US Marshals can enlist any civilian as a deputiy and form a “posse”! How is that allowed?
The US Marshal Service is almost as old as the Constitution itself. The office was created in 1789, although it didn't get its current name until 1969. The US Marshal Service is part of the executive branch of the government and serves as the enforcement arm of the US federal courts.
Since the inception of the office, marshals have had the power to enlist any willing civilian as Special Deputies, either as local hires or as temporary transfers to the Marshals Service from other federal law enforcement agencies.
The marshals were also given authority to swear in what's known as a "posse." This comes from the common law 'posse comitatus' which gave sheriffs the authority to conscript any able-bodied man to assist him in keeping the peace or to pursue and arrest a felon, as often seen in films about the Old West. "Posse comitatus" derives from a Latin phrase which means "power of the community."