Party questing frustration!

I don't understand how questing with other players works. Let me explain. Let's say I've already completed a certain quest to escort a group of NPC's to a given area while beaing attacked by the Charr. Then I meet someone who needs help doing that quest. We form a party and head out. We get to the location, there's the group of NPC's, but no Charr attack. The guy who needs to do the quest tries to talk to the Captain of the NPC group, but no response.

So what's the deal? I can't help him with the quest because I already did it? He can only do it if I leave the party? I've run into many of these types of situations already and its leading to a great deal of frustration. Am I doing something wrong? Is there a way to get around this system?

Try letting that person be the party leader, that mostly works. If not, then leave the area by teleporting away. Which i'm sure about.

I'll try the party leader thing when I get a chance... Is this frustrating for anyone else? It's nice that all I have to do is leave the group for them to be able to do the quest but sint the whole point of an online game to play WITH other people?

Part of the Guild Wars game engine is that they have a dynamic quest engine. The engine changes quests so that they fit each person in the groups history. So for some quests, if one person has already done it then it is not available.

it's happened to me as well a couple of times... we just leave the area, and when you go back, you get a new map, and things usually reset properly and work... I'm a lvl 7 ele/mo questing with my friend who is a lvl 2 warrior... it takes patience

Well, if you've already escorted those people to safety, why would they need escorting again?
The dynamic quest system ensures consistency so that paradoxes don't occur.
If that person wants to do the mission they have to be in a group in which no one has done the mission.