EDIT: The posters below are correct and I was incorrect. NPCs cannot be controlled. My advice was just for Quick Mode in general (PC characters).

You need to press space bar. There is a particular moment it needs to be pressed (timing can be difficult, so just keep on spamming the space bar until you exit Quick Mode).

Does Pool of Radiance allow you to control hirelings?

The Gold Box games I'm most familiar with is the Krynn series, and I know in that one there was a check that a Knight had to make in order to control NPCs. I don't remember how the Forgotten Realms games handled it.

I'm pretty sure you can't control hirelings in PoR.
Later games had the Paladin class, which enabled you to control hirelings.
In PoR you could to some extent control the hirelings' combat behavior by how you equipped them. If they had arrows equipped and a bow, they would use missile fire, which was useful for the lvl 3 Fighter, for example. Only the fighter hirelings are worth taking along, since the spell caster will just use up all their spells ASAP and not care about friendly fire, and the Thief will never try to backstab.