I'm fairly sure they were promised a better life than the one they had at home with their parents. If you're young, things are shit at home and then someone comes along saying they will take you to a better life then its understandable why they went. I am in no way saying what they did is right, all I'm getting across is why they might have been subject to.
The girl is called Shameema Begum. She was part of the 'Bethnal Green Trio' who joined ISIS, the others were killed shortly after being married to ISIS fighters.