Well, she has been a member of a few Black Ops, the government might have attempted to kill her and her old friends, and she managed to escape and meet up with the group. She might be a bit hostile at first, but knowing she's on the run, she wouldn't be too picky about who to stay with as long as they don't shoot her in the back.

Spies and traitors are not in my writing repertoire yet :p It's too hard to write a spy in a convincing and interesting manner.