I doubt she got into the car willingly per se, but was grabbed unawares and relaxed when she realized who it was. Unfortunately, for her. Should she have gotten in? No. Did she have a choice? I doubt it. I think it all happened too fast for her. Once in I think she thought she was ok because she 'knew' Sloot. I'm not sure if I ever heard anyone comment on her having met the brothers earlier though. Anyone?
Of the interviews I've seen, her friends only saw her with Sloot in the previous days. Apparently he was usually around them.
Since she was yelling & waving from the car, I wonder what the symptoms are of a combination of alcohol and date rape drugs?