Is there a script or method (I've only been using construction set for about a week now) that will allow only certain NPC's that are part of a faction to use a certain door?

I've got a world that has doors to each city pretty close to each other, and the NPC's of these cities want to use these doors for their travel purposes. Well.... in this case this city is like "The city of the Gods"
I don't want any normal nobody NPC coming into my city just to pass through to another one. Not to mention when about 20 NPCs are doing this in such a small place.. it gets rather crowded.

However I still want my NPC's that are part of my mod to be able to use these doors when I tell them too in order to go mingle with the "Mortal's" so to speak.

Create a new http://cs.elderscrolls.com/constwiki/index.php/Door object. On its settings, choose the 'Minimal Use' check box to keep NPC's from using the door.

Simple enough. But now if I tell an NPC to go from one cell to another, and that's the only door available to do so, will they use that door?

Probably not. But if you are not in the cell, it does not matter. The NPC's will magically appear where their AI package tells them they should be. Just like when you 'wait' and the NPC's are suddenly next to you.

Another method is to set the door's lock level to "Needs a Key" and set the ownership of the door to a faction common to all NPCs you want to use the door. As long as the NPCs are in the faction that owns the door, they can go through it without needing a key.