I highly, very very VERY highly recommend you set some ground rules and guidelines for this alliance. Limit 2 parties often fail miserably because of lack of coordination and strong leadership, resulting in mass confusion, frustration, and death.
You may think I am being overly dramatic in my presentation, but unfortunately, this is only the teaser trailer. You'd be surprised how many level 55 morons there are.
Be expected to have to hoof it the entire way too. Not too many people have the Vahzl gate key at that level yet. Understandable, considering trekking through that land isn't advised unless you're a mage or have plenty of items, but it's frustrating all the same. Also, make sure you knwo what you're up against. These NMs are very nasty, have awful AoEs, and respawn quickly. Set up a battle plan before you even leave town.
And most imporantly, and this sounds like a stupid thing to say but you'd be surprised, make sure everyone has started the quest! There's usually always at least one person who can't obtain the item because he didn't start the quest.
Do yourself a favor and don't make this more painful than it's going to be.