The sleepover happens in a basement. It's pitch black throughout the night and one girl transforms into a monster. The other girls know this because they hear everything and hide. The monster girl then taunts them throughout the night in the dark until the next morning, when she turns back into a girl. The other girls know it really happened though because they find scratches with hair in it on one the doors.
The book was a collection of perhaps six stories and I think the cover had some kind of generic monster on it. It was in English and was released in the mid 90s or before.