Who are the old Sarum bunch?

The Old Sarum Bunch were a tribe or gang that resided in the northern part of the country. The Cunninghams were a part of the gang, and Boo hung out with them before he got into trouble.

According to neighborhood legend, when the younger Radley boy was in his teens he became acquainted with some of the Cunninghams from Old Sarum, an enormous and confusing tribe domiciled in the northern part of the county, and they formed the nearest thing to a gang ever seen in Maycomb. (Chapter One)

Only once was Judge Taylor ever seen at a dead standstill in open court, and the Cunninghams stopped him. Old Sarum, their stamping grounds, was populated by two families separate and apart in the beginning, but unfortunately bearing the same name. The Cunninghams married the Coninghams until the spelling of the names was academic—academic until a Cunningham disputed a Coningham over land titles and took to the law. During a controversy of this character, Jeems Cunningham testified that his mother spelled it Cunningham on deeds and things, but she was really a Coningham, she was an uncertain speller, a seldom reader, and was given to looking far away sometimes when she sat on the front gallery in the evening. After nine hours of listening to the eccentricities of Old Sarum’s inhabitants, Judge Taylor threw the case out of court. (Chapter 16)