In one way the answer to this question is simple--the girls are making it up and have been since the beginning. But at some point in the story, the girls do begin to believe their own lies and they seem real to at least some of them.
The Crucible by Arthur Miller is set in Salem during the Witch Trials, and as early as the first scene we know that there is the rumor of witchcraft in town. Betty, Reverend Parris's daughter, seems to be stricken with some kind of mystical affliction, and soon Parris's niece confesses that they were in the woods with some other girls in town as well as the Parris's servant from the Barbados, Tituba. (Interestingly, Parris saw them there.)
While Abigail confesses that they did dance in the woods, she is adamant that they did nothing worse than that and Betty is not suffering from anything evil. She is equally adamant that there is no hint of witchcraft in this town. When no one else but the girls are in the room, Abigail is clearly the ringleader and she makes it clear that they had all better keep their mouths shut about what went on in the forest last night. We also discover Abigail's motivation for all of this forest activity:
BETTY: You drank blood, Abby! You didn't tell him that!
ABIGAIL: Betty, you never say that again! You will never--
BETTY: You did, you did! You drank a charm to kill John Proctor's wife! You drank a charm to kill Goody Proctor!
ABIGAIL, smashes her across the face: Shut it! Now shut it!
When Reverend Hale comes to town with his books, seeking out the devil in the form of witchcraft, Hale begins to ask questions. Abigail sees a way out and blames Tituba. It is a blatant lie told to escape any serious punishment and shift any blame to the defenseless woman. Tituba eventually folds and admits to witchcraft to save her own skin, and Abigail is quick-witted enough again to see that she can do the same kind of thing--give the people exactly what they want. At the end of the first act, we have an eerie scene in which all the girls begin a kind of trance-like chanting as they call out the names of their alleged accusers, probably the names of the women who have scolded them or that they disliked.
Abigail is the one who begins that chant, but she does not even have to explain what she is doing to the other girls; they are all afraid they will be punished for their actions and quickly follow Abigail's lead. Once the trials begin in Act II, we are clearer about the influence Abigail has over the girls. They are clearly moved by the power of suggestion, and Abigail plays them to her own advantage. When she says she sees a bird, they all see a bird; when she says she is freezing, they are all freezing. Mary Warren is forced to tell the truth but is unable to re-create the things she says she and the girls were faking, such as fainting or getting cold.
This is what I mentioned above, when I said the girls have bought into their own lies; however, their believing something does not make it true.
Abigail has made it abundantly clear that her only motivation is to get rid of Elizabeth Proctor in order to get John Proctor back, and she will obviously accuse anyone or do anything to make that happen--even drink a charm in the woods one night.
The very best evidence that all the girls' allegations are false is when Proctor mentions witchcraft in a private conversation with Abigail. She says:
Oh, posh! We were dancin’ in the woods last night, and my uncle leaped in on us. [Betty] took fright, is all.