The other people selected to memorize books who live as communal outcasts in the woods are happy, too. They are among those who recognized the dangers and the misery in the world of the present government. They are the ones braves enough to do something about it...not just sit in TV rooms and planning suicide attempts.
The "happy" live life more vicariously, but they are making their own choices. The ones who remain and live by others' rules are the miserable ones. See a pattern here?
All factors seem to indicate that most everyone IS miserable. Take Mildred's friends for example. They chat about t.v. shows and how awful children are. Then Montag comes in and quotes "Dover Beach", a poem that talks about how faith and beauty are disappearing in the world, and it upsets two of the women so deeply that one "sobbed uncontrollably" and the other lectures him that "silly awful hurting words...hurt people". Her friends seem very easily pushed over the brink into misery, which means that they are right on the edge. Mildred's suicide attempt is her going over the edge. The only thing keeping them distracted from their misery is staying busy with their t.v. walls.
Clarisse mentions how her friends are so busy with sports, car-wrecking, or other violent games that they don't have time to talk. She says that "they kill each other" and that "they all say the same things and nobody says anything different from anyone else." So, the kids are violent, busy, and robot-like. With all of the violence as a way to steer unhappiness, the kids are also distracted from being miserable, being in a society where parents don't love them and anything meaningful is filtered out.
But there is hope; Clarisse is happy. Montag starts the journey towards happiness. Other than that, it's a pretty grim reality for those in Montag's world.