Not sure how to do it via Webmin, but it's really not hard doing it by editing the /etc/httpd/conf/httpd.conf file.
If you use VirtualHosts, then yes, you can grant ownership to the specific user/group so that they can modify their webpage.
One option would be to enable Home Directories. You can enable home directories and change the paths. Just make sure that each user owns that directory. By default, once you enable the user home directory, it will be in /home/username/public_html. People can then access the site by using the username. For example,
http://mysite1.com/~joeblow
Yes, you can give permissions to the folder to the specific user and/or group to access the site. Or you can use the AuthType Basic directive to enable users/groups to login to access the site by granting them a password. YOu can also allow/deny from specific networks.