Who are these people?
They're prisoners. The Pit is a prison:
The Pit is a prison located in the ancient part of the world, which had established such a fearsome reputation that it became referred to as "the worst Hell on Earth".
Why doesn't Bane/Talia/Ra's Al Ghul (before he died)/Batman help to free them?
Bane, Talia and Ra's Al Ghul:
How could they free them, the prisoners were the ones that killed Ra's Al Ghul's wife (Talia's mother) and disfigured Bane:
Approximately 30 years before the events of The Dark Knight Rises, a young mercenary was sentenced to be imprisoned within this Pit for falling in love with the daughter of the warlord he worked for. Unbeknownst to the mercenary, however, the warlord's daughter arranged for his release under the condition that she take his place in The Pit. Also unknown to him was that she was pregnant with the mercenary's child and she gave birth inside the prison walls. She named their daughter Talia. Many years later, the warlord's daughter was attacked and killed by the other inmates. Talia, however, was protected by a fellow prisoner known as Bane, a man who spent his entire life in the prison.
Bane later aided in Talia's escape from the prison, which lead to his disfigurement through severe beatings. Bane was then treated by the prison doctor and heavily bandaged. Following Talia's escape from the prison, she located her former mercenary father, Ra's al Ghul, and he returned with the League of Shadows to exact revenge upon the prisoners who had killed the love of his life.
What better revenge than to keep them in there.
Batman/Bruce Wayne:
Bruce Wayne did actually help them: After he successfuly climbed out, he threw down a rope so the other inmates could climb it and go free.
How do the people down in the pit get food and water?
Read this scifi.stackexchange.com question.
Note: All the quotes are taken from Batman Wikia#The Pit.
