The Master of Shadows' Minions are lesser ghosts controlled and directed by the Master of Shadows.
In an effort to demonstrate his superiority over humans, the Master of Shadows directed groups of ghosts to attack several parts of New York City. Eventually, Egon Spengler correctly deduced someone else was controlling the ghosts. However, he came to this conclusion in a warehouse where several of these ghosts were throwing objects at him. Egon's observation didn't go unnoticed and the Master sent away the ghosts then introduced himself to the Ghostbusters.
Soon enough, another group of minions manifested at a roller derby on 23rd and Lex. Winston Zeddemore himself was accosted by a pair of haunted roller skates. Once Winston got ahead of the ghosts, he shot the skates and was thrown forward. He got a clear shot and confined the ghosts while Ray Stantz and Peter Venkman trapped them. A third group of minions, this time a mere trio, attacked a city near a candy shop and water fountain.