Aillén O’Brian came to the Nest because they needed someone who know how to maufacture SINs and forge other stuff. With most people SIN-less, that’s an important skill. Aillén lived at the Plastic Jungles at the time, but when she tried to steal Neil’s commlink, she was kicked out by the leader of the community she was living with, despite the intervention of her foster-mother.
The offer the Nest had to make was way better than living (and probably dying) on the streets and so she agreed to come with them, after some discussion.
Aillén has been manipulated by unknown means into giving away several thousand credits. She’s the second person in the Nest this has happened to after Liz. Neil found the same unusual brain activity Liz showed, vaguely similar to the victims of the Matrix Surgeon.