People seem to have a lot of interpretations of this sentence, but I think they inflated it a little. In most of Bradbury's work, the simplest answer usually makes the most sense.
"But most of the houses, darkened, were sleeping already and there were a few light-less places where the occupants of a dwelling sat talking low night talk on their front porches."
What he meant was that on the porches of the few houses with families that were still awake, those families (or parts of it) were outside talking. The "occupants of a dwelling" would be the people who live in that house, and they, like a few of their sleepless neighbors, are out on their dark porches, speaking quietly to each other.
To make it clearer, using "a dwelling" emphasizes the family unit. Saying "those dwellings" lumps everyone outside into one group. I haven't read that story in particular, but emphasizing the tiny pockets of people gives the image more of a paranoid feeling(for me). What are they talking about? Do their sleepless neighbors agree?