Party1 is always the party leader so for all members in the group, that is a constant.

I *believe* that party2 is always the toon itself (for the non-leader toons).

Party 3-5 is then the old reverse order of original creation date.

So, the net-net here is that the order is not constant for all five toons. That said, the group order if you use a consistent set of 5 toons will be the same everytime. So, I know what order my priest is in my group and code macro's accordingly so that I'm healing in "order" from my tank (party1) screen. For my shaman, I had to change the order a bit:

Code:
Leader's Screen  Priest PartyX Target  Shaman PartyX Target
Main             Party1                Party1
Priest           Party2                Party3
Shaman           Party3                Party2
Mage             Party4                Party4
Warlock          Party5                Party5