Hell Is Not Other People
To make this short, Hell is not other people, because Hell is people like yourself. In other words, hell is yourself.
The fact that everyone refuses to leave despite the door being unlocked is basically a direct statement on a person's need for other people, regardless of who they are.
The unlocked door itself symbolizes the potential for change, while the people like ourselves in the room symbolize the reflections of our negative traits we see in others, whilst the two symbols combined are themselves a symbol of the human potential to escape from one's self, from one's already defined qualities, into something different, and possibly better.
In short: the room is you. The other people in the room reflect your negative traits that you see in others, and you within the room represents your tendency to self-aggrandize and cast your worse parts onto others, as opposed to taking responsibility for them yourself. The unlocked door represents the idea that "no man is an island."
I think this play is about the Hegelian view of self-conscious. No one can understand that they exist without coming into contact with another self-consciousness. And when the only knowledge of your personhood comes from people who tell you you're worthless, that is what you will come to believe. You have no way to know otherwise.
Inez's insistence that deeds are the only proof of worth
Estelle not feeling real when she can't see herself
Garcin being unable to leave when the door to Hell is opened: without Inez giving him the harsh truth, he won't be real anymore, and his desire to change her mind because in her belief is his salvation