Divvying up punishments on a per head basis is not a valid option, like at all
Well it seems I have a fan, quoting my magnificent words for the whole world to read
But what is not valid about it? Like I said with the bank thief analogy, if 100 people steal $100,000 together, then they must pay back $1000 each, since each of them is responsible for 1% of the crime that was committed. The same logic can be applied to any crime: if a certain amount of damage is inflicted, then each perpetrator is responsible for a portion of the damage.
I'd hate for the guy just "standing around" to be punished unfairly
Well, that would complicate things quite a bit. I suppose you could list who is most responsible and least responsible for a rape, and hand out different jail sentences accordingly. For example, the one who brought everyone together, stalked the girl down and executed the initial attack would be more to blame than the rest of them, and thus he should receive a longer jail sentence.
But like I said many times before: the murder basically overrides the rape. If you rape and then murder someone, it becomes just a case of aggravated murder. And murder is easier to quantify than rape, since you don't have to worry about the whole "person X suffered more than person Y" issue, because all victims of murder suffer equally, so you can very easily do a "head count" and quantify it based on how many murders took place.