07 Jul 2009 at 20:59 - 10
As Snoggy says, there's two issues here - one to do with "scaling" and one to do with allocation.
When it comes to allocation, drops are randomly allocated to party members. It doesn't matter whether the party member is a henchmen, a hero, or a human, they have the same chance to get a drop. (Which makes me wonder why people look for "leechers" when heros or henchies would do as well.) So, in Magma's example, you would have a 2.5% chance of getting the drop whether the other party member is a hero, henchman, or human.
Loot scaling, affects the total amount of drops, not how they are distributed. Iirc, if you go into an area with a group of 8, you will get a certain overall number of drops. If you go into the same area with only 4 people, you will not get the same total number of drops, but then again, you will not get only half either. You will get some ratio in-between which is "scaled" depending upon the group size and the speed with which you kill things. If you go into that same area solo, you won't get all the drops, but you won't get only 1/8th either - again it will be "scaled".
So, the OP's post isn't exactly clear, but the end result is that any party members affect loot scaling and all party members can be assigned drops. It doesn't matter whether they're human or not.