Use htpasswd to create extra instances of your users in the .htpasswd file used to provide access to the DAV directory. As in other hints, use this command to add users to the .htpasswd file:
htpasswd -m username /path/to/.htpasswd
Then run the command again, prefixing the username:
htpasswd -m "domain_name\username" /path/to/.htpasswd
So, if my computer is example.com, my user is bill, and the password file is in /Library/WebServer/Documents/dav/.htpasswd, I'll run these two commands, entering the same password for each:
htpasswd -m bill /Library/WebServer/Documents/dav/.htpasswd
htpasswd -m "example.com\bill" /Library/WebServer/Documents/dav/.htpasswd
Make sure you run this command on the correct file -- you may have more than one .htpasswd on your server, or you may have used another name entirely. The -m uses a better encryption scheme; it's optional. In the second version, I used double quotes because I wasn't sure what the backslash would do; maybe they aren't necessary.

