Garcin's exclamation that "hell is other people" is an expression of his newfound realization that his eternal existence will be the punishing state of a triangular relationship with Inèz and Estelle. His only way of defining himself will be through their perceptions of him. There will be no redemption for him or for them; their damnation is that the sum total of their respective existences has brought them here and no further. What they share about themselves will define them forever in the eyes of the others; they comprise the rest of the world, and are fallible and narrow in what they can perceive. It shrinks the essence of these three people in a way that can never be changed.
It can be argued that one can hide from oneself indefinitely by choosing not to reflect on one's past. But once the three people in the room share their stories, they can't retract or revise them. The die is cast, so to speak, and they become only what the others construct about them based on what they hear and observe. This is emphasized when Inèz tells Estelle that she will be her mirror; she will be the one who tells her what she looks like. In this way, Garcin understands, too late, that in telling his story he has ceded control to the "other people," Inèz and Estelle.