There are several examples of "guilt by association" in Act Three of The Crucible. The first comes when Proctor and his friends present a petition to the courts, where close to one hundred townspeople signed their names, asserting that the arrested women were not witches, but instead righteous, god-fearing people. The courts, instead of being impressed that so many people were willing to testify to the good nature of the accused, instead assume that they must also be guilty and covering for the witches. They think it is some sort of scheme to get the guilty women out of jail and back causing havoc in the town. So, they arrest all of the people that signed the petition.
The second example is the unnamed witness that declares he heard Thomas Putnam say that he had prompted his daughter to accuse a man of being a witch, simply so that he could buy up that man's land. Instead of believing the witness's testimony, they assume that the man is lying, and that he too should be questioned. To top it all off, Giles Corey, who presents the information, is arrested too, merely for relaying the information.
So, those are a couple blatant examples from Act Two where innocents are assumed guilty simply because they associate and defend the women and men who have been arrested. It is pretty sad. In these cases, none of the accusations of guilt were valid or true. Now, if Giles Corey had been at the same party where a bunch of witches had been performing spells, then maybe the entire "guilt by association" theory would hold some credit, but in this case, there was no proof or logical attachment to the actual guilty parties.
That can help to get you started; good luck!