Depantsed though that's probably a given.
water + white top = nice
A bad guy whos... sharking everyone? like the japanese.
lifted skirt for whatever reason... maybe even contrasted with some exhibition of one of the girls freeballin'
a guy possibly of the boyfriends or good friends walks in on one having a shower.
Thats some basic ideas
I also must add, with the towel ending up naked idea, how old are the pp girls? Are they still in school or in college? because I was thinking, instead of a slumber party, in a changing room possibly at school
read this for inspiration
http://www.thatsembarrassing.com/story-849.htm