While you'll want to research the play further to gain a complete understanding of how "mob mentality" is used in Arthur Miller's The Crucible, there are a few sections of the play that stick out. Take special note of Abigail as she incites the mob to act rashly and illogically on numerous occasions throughout the play.
After being caught dancing with a group of girls in Act I, Betty falls into a coma-like state. The villagers gather, and once she wakes up, they begin claiming witchcraft:
ANN: (Entering) The psalm! The psalm! - she cannot hear the Lord's name!
PARRIS: No, God forbid...
ANN: Mark it for a sign, mark it...! (Rebecca Nurse enters.)
PUTNAM: That is a notorious sign of witchcraft afoot, a prodigious sign.
ANN: My mother told me that! That they cannot bear to hear the name of...
PARRIS: Rebecca, Rebecca, come to her...we're lost, she suddenly cannot bear to hear the Lord's name.
ANN: What have you done?
This is the first example of mob mentality in the play. As soon as Ann suggests that Betty's freak-out is a result of the psalm, the rest jump in and decide it must be witchcraft.
A short time later, Abigail becomes hysterical and blames everything—the dancing, Betty's condition, her laughter in church, etc.—on Tituba. The "mob," consisting of Hale, Parris, Putnam, and others, decides that Tituba has been consorting with the Devil.
ABIGAIL: I always hear her laughing in my sleep. I hear her singing her Barbados songs and tempting me with-
TITUBA: Mister Reverend, I never-
HALE: When did you compact with the Devil?
TITUBA: I don't compact with no devil!
PARRIS: You will confess yourself or I will take you out and whip you to your death, Tituba!
PUTNAM: This woman must be hanged! She must be taken and hanged!
The dramatic element of the scene aside, this passage is almost comedic. Abigail, who has been caught dancing at night—something that is clearly against the rules in their society—passes blame onto Tituba. Instead of looking at this rationally, the mob decides that as punishment for allegedly tormenting Abigail, Tituba must die. Again, Abigail is the mastermind behind this mob mentality.
One final example comes later in the play when Mary is testifying to the court. She confesses that the girls were all just pretending that they were bewitched when Abigail turns it back on Mary, again provoking the mob mentality of the crowd:
ABIGAIL: (Backing away to bench and sits. Clasping her arms about her as though cold.) I...I know not. A wind, a cold wind has come. (Her eyes fall on Mary.)
MARY: (Terrified, pleading.) Abby!
MERCY: Your Honor, I freeze!
PROCTOR: They're pretending!
HATHORNE: (Touching Abigail's hand.) She is cold, your Honor, touch her!
MERCY: (Rises. A threat.) Mary, do you send this shadow on me? (Sits slowly.)
MARY: Lord save me! (Susanna rises looking at Mary, then slowly sits.)
ABIGAIL: (She is shivering visibly.) I freeze-I freeze. (Mercy hugs her as they shiver.)
MARY: (With great fear.) Abby, don't do that! (Proctor crosses to her, grabs her.)
DANFORTH: Mary warren, do you witch her? I say to you, do you send your spirit out!
Yet again, Abigail inflames the crowd's fear of witchcraft; however, on this occasion, she also provokes Mary's fear of being accused of witchcraft. The other girls follow her lead, and after a lengthy confession from Proctor about his affair with Abigail, Mary finally gives in and accuses him of consorting with the devil.
There are other examples of mob mentality throughout the play, but these three should get you started. It is a common thread throughout, especially when Abigail is involved. Pay close attention to how she is able to rally the mob however she sees fit through her actions and accusations.