Would you consider a woman slutty for letting a group of guys have their way with her?

I would not call her slutty just troubled. Some will excuse this as experimentation or a high sex drive but it's probably something more than that if you're willing to put yourself in a dangerous situation on a basic impulse (and yes group sex like that is dangerous both physically and socially, it can bite her in the ass years later and the fact that she didn't stop to consider the repercussions is why I say it's a troubled person.