On the surface, Mary Warren and Abigail Williams seem to be friends. They are the same age, and they were both present with Tituba the night before the play begins, attempting to conjure the spirits of Mrs. Putnam's dead babies. Further, Mary now occupies the position in the Proctor household that was vacated when Elizabeth Proctor discovered the affair between her husband and Abigail seven months ago. After she fired Abigail, she hired Mary.
Under the surface, however, it does not seem like Abigail and Mary are really very friendly. Mary is introduced as a "lonely girl" (implying that she has few real friends, if any) and she and Abigail are at odds right away about whether or not to confess what else they were doing in the woods. Mary wants to tell, but Abigail will not. In fact, Abigail dramatically threatens to come to any of the girls at night, with a "pointy reckoning," and "make [them] wish [they] had never seen the sun go down" if they tell. Threatening with knives is clearly uncommon behavior between friends.