Jo first stumbled across the team when she was an aspiring journalist. She got a visit from Adam dressed as a gas meter reader and was immediately suspicious something was up. Her natural spy instincts lead her to follow Adam to an MI5 safe house, where she got caught up in a shoot-out and her quick thinking helped the team track down the perpetrators.
As luck would have it, Jo was looking for a job, and MI5 were looking for new recruits. Adam grabbed the chance and offered Jo a job with the team.