The main truth exposed by the girls in "The Crucible" is that the girls pretended to be possessed by witchcraft, according to About.com. SparkNotes also mentions Abigail as the ringleader of the group, urging the girls to admit and disclose nothing to the elders about dancing and engaging in witchcraft in the forest. Wikipedia notes that Abigail threatens three of the girls if they reveal what happened in the forest.
CliffsNotes mentions that Abigail drank blood and cast a spell to kill Elizabeth Proctor, John Proctor's wife. Betty Parris, one of the girls in the group, fakes being in a trance around adults, but awakens when talking with Abigail. Many of the people being accused of witchcraft by the girls had negative encounters with them in the past, according to eNotes.com. Abigail accuses Elizabeth of witchcraft out of revenge and jealousy. Abigail was once a servant in the Proctor house, but Elizabeth sent her away after discovering that Abigail and her husband had an affair.
About.com mentions Act III and how John Proctor urges Mary Warren, one of the girls, to admit the lies concerning claims of possession and witchcraft. When she admits to the court that the girls had been lying, the girls respond by acting as if possessed by Warren. To save herself, Warren then accuses Proctor of bewitching her, which results in his arrest by the court.