The Dungeon Master's Guide explains that DMs
Do not award XP for creatures that enemies summon or otherwise add to their forces with magic powers. An enemy’s ability to summon or add these creatures is part of the enemy’s CR already. (You don’t give PCs more XP if a drow cleric casts unholy blight on them, so don’t give them more XP if she casts summon monster IV instead.) (37)
Does this mean the DM should not award XP for enemies added to the battlefield via spells and magic powers even if those spells and magic powers have been employed prior to the encounter?
Further, what about enemies added to the battlefield via means other than spells and magic powers yet that still expend or exploit the creature's resources? A druid's animal companion and a paladin's mount are among the "magic powers" obviously included in the creature's Challenge Rating, but what about cohorts and followers via the feat Leadership (DMG 106) or animals that have been purchased using the creature's NPC Gear Value (DMG 127)?
In short, did I miss--in the DMG or elsewhere--the section addressing creatures using their magical or nonmagical resources to have more creatures on their side before an encounter begins? If I didn't miss that section, is it a reasonable house rule to extend the rules above from the DMG to cover such encounters?
(By the way, I know that Challenge Rating and Encounter Level are guidelines--wildly subjective and often inaccurate guidelines--, and therefore I'm unconcerned with an evaluation of the encounter's perceived difficulty. I know I must adjust Challenge Ratings and Encounter Levels based on the characters and circumstances.)
I'm designing an encounter with a hag covey (MM 144), and when 3 hags are within 100 ft. of each other one can employ the spell animate dead [necro] (PH 199) as a spell-like ability 3/day as a 9th-level caster. The covey's had both time and access to corpses. That means 12 9-HD CR 4 skeletons (the group of which is an EL 11 encounter) can be present alongside them.
Further, one of the hags is a cleric who can cast planar ally [conj] (PH 261), and the hag's NPC Gear Value is sufficient to add to the enemies a 12-HD elemental or outsider to perform a "long-term task, one requiring up to one day per caster level" (PH 261), and that outsider or elemental could reasonably include a glabrezu (MM 43), kelvezu (MM2 61), rukarazyll (MM2 181), or an extremely liberal arcanaloth (MM2 203) with its at-will spell-like ability shapechange [trans] (PH 277-8). Yeah, I know--the Monster Manual 2 has some crazy stuff.
Also, all 3 hags possess sufficient class levels to take the feat Leadership and, narratively, should have it. And, narratively, their cohorts should be present during their encounter with the PCs, and their followers should be nearby.
Don't worry about the PCs' survival. They're tough. But I am worried about rewarding them appropriately if they defeat the covey. The hag covey has a lot of baggage but mechanically only generates treasure appropriate for an EL 18 encounter (3 CR 15 creatures) unless their animated dead, planar ally, cohorts, and followers all count toward that EL.
I know I can totally ignore the entire Encounter Level calculation and instead use the skeleton horde and the planar ally as one encounter, the cohorts and followers as a second, and the hag covey as the boss fight, but I'd still like to know the actual rules.