Determining CR / XP when NPCs joined the fight?

[Nebulae]

My intrepid band of adventurers recently finished a campaign where three power bases were fighting for supremacy. The final boss fight lasted over an hour of in-game time and had multiple moments where one of the party members would run away and return with reinforcements from one of their allied sides.
At this point, I don't know how to calculate XP for the session. If I take the CRs of the enemies and subtract the CRs of the allies, I have a negative CR for the entire battle. My players suggested that I count the CRs for the enemies and disregard all of the allied NPCs, as they did not survive the battle, which would give a 30+ CR. Suggestions?

Battle participants:
4 party members (1 died/3 survived)
3 officers from side #1 (enemies, 2 died/1 survived)
1 officer from side #2 (started as ally, switched to side #1 when he discovered the party had murdered the side #2 leader, died)
12 foot soldiers from side #2 (allies, died)
2 adventurers from side #3 (died)
4 defenseless townsfolk caught in the crossfire and used as human shields (died)

[Aranian]

The awarded XP should generally not be reduced if the PCs had outside help. So in this case they should get XP for all officers of side #1 and the officer from side #2. Then just sum up the XP for these foes and divide them by four and award each character that amount of XP.

You could, as GM, decide to spontaneously reduce awarded XP if the PCs steamrolled an encounter because of outside help. You could, for example, count each foe as one CR lower than they actually are for XP calculation.

[Chemlak]

NPCs earn XP, too.

Take the total of the "enemy" XP, and divide that by the number of non-cohort (which I believe is all of them) NPCs plus the number of PCs. I make that 18.