Or delete the folder /home/site##/users/username/web and sim link it to /home/site##/web like this: ln -s /home/site##/users/username/web /home/site##/web
Be careful with this. I think if this is done, when you go to delete that user, you'll delete the contents of main site with him/her. Better to just educate your end users IMHO. Tell them to set their ftp clients directory to /web. For Dreamweaver, use ../../web.
HTH, j