It would be interesting to look at the two Reverends and explore how different they are. Parris is worried about reputation and will do anything to be sure his reputation is kept intact, including convincing himself that witchcraft is occurring. Reverend Hale, on the other hand, intitially believes in the witchcraft because he has studied it and truly believes it is happening, but when he sees the truth he admits to it and, putting his pride aside, tries to convince people to do what he knows is right.
When I first read the play, I, too, was surprised by the mob mentality of the girls and the community. One suggestion would be to look at the people who remain good over the course of the play. Given that not many characters do, you could contrast their behavior with those who are not "good."
What about writing about John Proctor. Why does he allow himself to be killed? Is he motivated by selfishness or is he acting based on principle? What sort of a personality drives a person to allow himself to be killed rather than to tell a lie?
The "group mentality" of the young ladies is not surprising, except for its consequences. The willingness to go along with an impossible story - to accept rumor and fancy as fact - is characteristic of the personality of "the mob".