The Gang is what Todd, Curtis, Jenny and Hannah call themselves as they join up to find and battle The Book of Pure Evil.
For a while Atticus was part of the gang, but he got kicked out when the others found out that he was a satanist.
Jenny's dad Pat also tried to join the gang, but only succeeded after he used the Book to become a skin changer and disguised himself as Devin, the School President. His membership was short-lived due to Jenny finding out and him dying shortly after.
In episode 2x13 Curtis disbands the gang after Hannah died as a result of Todd sending the book away to an unknown location.