How are quest items handed out? If I'm doing a quest that needs 100 items and I join up with someone who also needs the 100 items - do we both get it if the quest item drops or does it rotate?
It depends on the quest, some quests you get quest items that can cannot be shared/traded like the quests for lvl35,37,39(profession quests) or like the quest for Abanded Camp.These both ppl get items np.The other type of quests,show up in your items slot,can be sold/traded,
and therefore random/turn based depending on party setting.
quest rotate/random per person's your partying and multiple quest with same mob screw the drop of the other. now quest like Giants Cave quest & ToI quest are better done in party groups since the drop rate its so bad that you may have better change of getting the item with party members.
It depends on the type of quest you are doing. If the quest is a team based quest you will both have the item (depends on how many people in party though, cause for the TOI quest For exemple if you are a full party the quest items become more rare). If your quest is a one person only quest it is the one who will have the last hit on the mob that will get the quest item.
The Random/by turn party set up that tseralyn is talking about has to do with loot (sometimes also quest items like the sealstones in Catacombs/Necropolises) that drops from mobs.