It's no problem, frequently get new people who are unfamiliar with running a web server, new to Unix, etc - it helps to search the forum a bit first though.
The first thing you should know is that a user is just an way for the operating system to keep track of things a person does. Also, the user is assigned a home directory. This is his diskspace, to store his files, without them getting in the way of other users or important things like the OS and other programs. If you are familiar with Windows XP, this is analogous to the Documents and Settings directory.
You don't associate a user to a domain explicitly:
So what you want to do when hosting a domain that is to managed by another person is to add a user to the system for them to use. Then, when you create the domain, or if you already have it created, you can use the Web Panel to say that the files for the domain are located in a directory inside that user's home directory. The Web Panel calls this the "Web Directory", though to the web server software its "DocumentRoot". The web server software will then know that http://domain/somepage.html is actually /home/username/domain/somepage.html on the filesystem.
So once you have the user and domain setup in the Web Panel, its time to upload the files. You do not get a FTP or shell "superuser" that has control over other users files. Once you setup a user, in order to create, edit or delete the files in that users home directory you should always login to FTP/shell as that user, and not as any other user on your plan. This is because the filesystem, the user that creates the file or directory "owns" it and gets to decide what others can do with it; this is not compatible with the notion of "superusers" and "subordinates". However, since you are the one with the WebId account, you can always retrieve the passwords for the users on your plan if you need to login as one of them in order to manage the files.
One thing to note is what you will see if you log into FTP. For a regular FTP session, your FTP client is restricted to your home directory, and your FTP client will call it "/" instead of "/home/username". However with secure ftp, the client will be told the full path on the filesystem - including part of which is a mount point - for example "/home/.mountpoint/username". For the most part you only need to use the "/home/username" version for configuring stuff. Anyways, once the user logins into FTP, there should be a directory named after the domain in which to place the files, just as you have probably been doing with your own user until now.
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