who is the old sarum bunch and what is their puporse for talking to atticus ?

Basically they come to the jail to lynch Tom Robinson. THey do this in full view and intend to take their chances with a biased jury later. They want to take revenge on him themselves and they do not want to risk having him be acquitted in a jury trial. It is a little girl, Scout, who calms Mr. Cuningham down and helps disperse the tension.