My guess is that she knew one of the younger members, probably thought of him as a boyfriend. Getting into the car then, especially if she wanted to drink or smoke “stuff” with him was probably OK from her point of view. I’ve worked with a couple of Foster teens who were in sexually abusive situations, either prostituted or with multiple partners, that started with meeting up with a boyfriend.
Your explanation makes sense. Disgusting pigs! Foolish girl.
But, as I said, the article is so poorly written that it is confusing. If the school authorities knew to report her as missing when one of them saw her getting into a car, why would they do that if she was just getting in normally as she might with a parent? What told them that she should be considered missing? Strange.