1st off, when dealing with a VERY large group, increasing monsters is rarely a good idea. turns already are going to be cumbersome so adding more monsters are rarely a good idea.
(quick glimpse through the adventure, not a detailed analysis, so general advice rather than specific advice.)
I would suggest leaving it as is at the beginning, allow them some easier encounters at the beginning to feel them out, if you feel you need to spice up certain encounters like key battles, consider upgrading something by a hitdice or two.
if they are having too easy of a time of it, promote an enemy every other encounter (an extra HD, a +1 to hit, and better armor also gives a good bulls eye for their better spells), and/or force them to delve deeper than the adventure would assume they would at a stretch. the main bad guy is a leveled druid so he would understand the tactic of sending minions to disrupt a casters 8 uninterrupted hours of rest.
if they are heavily overpowered for the adventure (experienced players using optimized characters) I would suggest allowing them to simply rip through the adventure, and prep something a bit beefer up front for them for their next adventure, if you want to stick to premade adventures at this size of a group, if they have the experience and characters they should be treated like a 4 man party of 2-3 levels higher.