I have a RAID 1 configuration and was just wondering if I were to add a external usb drive if it would break the array or do any thing performance wise???

Is this a hardware or software RAID? An external USB drive shouldn't affect the existing RAID in any way. Your system should see it as a removable drive.

It is a hardware RAID. Now my question would be is: would domain users be able to get this usb drive mapped on their end? Or do you prefer NAS?

They should be able to providing you share it and add the user's accounts to the ACL on the share first. I would prefer a NAS I guess but I have a pile of extra PC's here at home (I'm a computer/network geek by trade) so I use one or two of them for backup storage...pretty much the same thing as a NAS right. If I didn't have a pile of extra PC's I'd probably invest in a RAID capable NAS. I was looking at one the other day I liked. It had 5 drive bays, hotswappable. If you plunked two drives in, it formed a RAID 1. If you added a 3'd, it changed it to a RAID 5. If in the future you find you're running out of space, you buy a larger HDD, put it in slot 1, it becomes added to the RAID. Add a second HDD in bay 2 and it rebuilds the RAID on the larger disks. Then you replace the remaining disks with your other new HDD's and bing bang boom, it rebuilds, sees that the drives are bigger and automatically makes allowance for them. Seemed like a pretty sweet unit to me.....hmmm....and x'mas is coming......*grin*