OK, let me confirm what you are trying to do. From your description it could be one of two things.
1) you have several people with notebooks that want to share between themselves via the NAS (not the best way of operating).
2) you have several people with notebooks that access to the NAS to store and/or backup files.
Which of those two is what you want to do?
You also state you are trying to use webDav. What webDav client are you using on the notebooks? I ask because the win implementation of webDav in windows explorer isn't exactly standards compliant to use over the internet.