As for priority: it will calculate a number for each unit by:
player himself = 800
people in your subgroup = 700
other subgroups = 620-690
pets = -200
individual priority list members = +200
priorized classes = +200
priorized groups = +200
and sort the list, highest number first. Hope that helps.
Any chance of one day (far down the road) getting those values adjustable as part of an advanced options menu? For example, I have no reason whatsoever to prioritize my own group over everyone else. In fact, it could be detrimental.