I would imagine the only other option is to install a CGI application or the like that acts as a file manager, because the file system just does not support "concurrent owners" and so at the application level you would have to pretend you have concurrent owners but actually only have one.
For example, you could have a WebDAV "incoming" directory and have the users upload gzip files and then have a CGI script pull the gzip file using HTTP from the WebDAV and automatically uncompress the gzip to the right spot, etc. The CGI script would run only as one user which would own all the files on the domain (except for the ones in WebDAV folder which would be owned by the web server). Thats just a crazy idea of mine though.
If I were you'd just give each user their own FTP account and tell them to upload files to a world-readable or group-readable directory then have a third user (password-protected CGI script) copy files from that directory to its own directory where the domain is actually served from.
PS. You don't have to give out your uber-login. Lets say your uber-login is 'superman'. What you do is create another user called 'clark', and then have the domain served from the 'clark' home directory instead of the 'superman' home directory, and then tell your people to both login to FTP as 'clark' - and if you need the password for 'clark' you can always have it e-mailed to you using the web panel.
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