The Inquisitorial Squad was formed by Dolores Umbridge during her time as High Inquisitor at Hogwarts. Umbridge herself effectively led the squad, but she appointed several prefects, mainly Slytherins, to help enforce her rules.
These prefects included:
* Draco Malfoy
* Pansy Parkinson
* Gregory Goyle
* Vincent Crabbe
* Millicent Bulstrode
While they all had authority, it was ultimately Umbridge who held the ultimate power and made the final decisions.