I don't recall slick 2D having an entity manager, only some of its extensions that have and a few tutorials that explains how to make one.
My advise, make your own, it will be alot easier to understand whats going on, than using someone else's system.
For your crowd manager(npc-director), make it an entity, and make all the npc's entities too, have the npc-director create and remove npc's first thing per loop, maybe using an npc pool, npc-director could assign them behaviors too.
Then have the npc's control what they do on their own.
This is what i would do based on your description.
But its up to you, to decide how it should work.