Actually, what is happening is FTP is used to upload files to the home directory of a user. A home directory just a storage location set aside for that user.
Then, a program called a web server is actually running the web site. This web server is configured to look for the web-accessible files in a directory owned by a particular user.
No, the FTP user that can manage the files is the "CGI user" as specified in the settings for a "Fully-hosted" domain or subdomain, and you get to choose which user is the CGI user.
So this means you do not have any domains or subdomains that specify them as a CGI user. If you intend for these people to upload files for the same domain/subdomain, they will need to share the username/password for the CGI user.
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