In Shakespeare's Macbeth, like a witch, Lady Macbeth conjures spirits in Act 1.5. She asks the spirits to "unsex" her: turn her from a female to a male so she can be evil and ruthless enough to kill Duncan. This connects her to the witches in the play, not only because she is conjuring spirits, but because the witches appear to be women, yet have beards (Act 1.3). They are androgynous, as Lady Macbeth would like to be.
She is also similar to the witches in her use of lying, false appearances, equivocating. The witches serve as catalyst for the conflict by predicting Macbeth will be Cawdor and king of England, and then later reassuring him of his safety and success by predicting that he cannot be harmed until Birnam Wood moves, and that he cannot be harmed by a man born of woman.
Lady Macbeth acts, and orders her husband to act, as if their is nothing wrong when Duncan comes to visit their castle. She lies about what's wrong with her husband after he sees Banquo's ghost at dinner. She acts surprised when Duncan's body is discovered.
Put simply, Lady Macbeth is considered to be a witch-like person, because she is a witch-like person.