Absolutely you can invite church staff or pastors . . . unless you think it will impact the transparency and vulnerability of the group. Mentoring only works when the guys in the group are fully open with each other and willing to allow others to speak truth and give honest feedback.
Just keep this in mind as you invite church staff or pastors. If you don’t think they will be fully transparently and vulnerable because of their positions or if you think the other men in the group won’t be fully open with a pastor in the room, you may want to hold off.