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.

My idea's:
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.