I am running the AP with a large party, 9 characters. With there being so many PCs in the group, they just entered the Temple of Demogorgon and have just turned 7th level. I usually add a couple of extra creatures for them to fight, (raise the CR so the party doesn't fall too far behind). I determined I could not do that with Olrangru's mates because the party had their a**es handed to them when the demons popped in to kidnap Urol. After the party entered the temple, I changed the mirror portals, but they all entered one portal. This put them entering the mates' lair one at a time. The battle didn't go very well for the PCs, (they won, but used almost all of their resources). If they make it out of the temple, what can I do to make sure they are not overwhelmed?
I wouldn't be too concerned. I am also running a large party, though not as large as yours. The PCs will fall behind by splitting the XP more ways, but they make up for that in the sheer amount of damage they can do (or healing they can suck up) in one round. Plus, the farther behind they get, the higher those XP awards will be. The game is self-balancing that way.
If your party does not work together well, you might want to selectively edit some creatures out of the STAP and replace them with a larger number of lesser creatures. The vrocks in the next chapter were vicious.
If your party works together too well, you might want to add some meat-shields (lower level henchmen) as a distraction to beef up some encounters.
1) Are you giving your PCs enough XP? By the time they reach the Temple, the adventure is counting on them being 8th level. If you weren't adding any monsters to the encounters in the Temple, they should have been fine. If you were, though, even just to keep their XP values up, it may have created to large a challenge.
2) Are you giving your PCs enough treasure? This is a bit harder to track with a larger group, but by 7th level they should have the guideline of 19000 gp in wealth each, even if it isn't ideal equipment. If you've added monsters for level gain but no treasure, your PCs will be overwhelmed.
3) Are your PCs working together? Are they concentrating on one foe at a time, protecting those who need meat-shields, and coordinating their attacks? Are they utilizing flanking, trips, bull rushes, and/or battlefield control magic? Or is it more of the "spread out and kill what you can" type of tactics? In a larger group, the second is more common, but far less effective. Mechanically, you could encourage the use of Teamwork abilities (PHB II, Heroes of Battle), but it's mostly just a matter of sitting everyone down and telling them that if they are to survive the campaign, they need to work together.
I also am running a large party, 7 characters including a Druid with a Rhino, and they are not have too hard of a time. They also went through the temple at 7th level. Once they figured out the the paladin could bless their weapons, the demons were a lot easier.
In all, I just add one or two enemies to each encounter and it seems to be pretty even. They almost lose the tough fights, but otherwise seem to move right along.
1) Are you giving your PCs enough XP?
2) Are you giving your PCs enough treasure?
3) Are your PCs working together?
1) I have been adjusting the XP I awarded for the additional creatures they have faced.
2) I beleive they are close to the 19000 gp per character, but I will take a look before the next session.
3) They do not work well together. The rouge likes to run up and take advantage of the sneak attack, did that to the T-Rex, and was swallowed, still has not learned. They have a dwarf fighter whom they put 2nd in line for his 93hp at 7th level. He always seems to block the way for the rest of the party, 20' move vs the barbarian's 40'. With them, they don't seem to care how the battle goes, as long as their charcter strikes the killing blow.