I'm currently a group therapist at my hospital. Know your boundaries and how to express yourself. The mental health field is great for INFJs since this is basically our home turf. That said, this is where clients can also potentially confuse our emotional landscapes. Take care to constantly check your well-being and use an authoritative but friendly tone.
Oh yeah, and stay focused on the problems they're there for. I've often let sessions go over time because I got wrapped up in the conversation too much. Know where you need to go with the client.