We use the Webmin user management functionality found in web-lib.pl (and all of its various modules). This puts user data into files found in /etc/webmin/
The files are simple key=value configuration files. Nothing complicated at all.
What are you trying to achieve? There might already be some functions you can call in the Webmin::API Perl module, or in one of the various command line tools we provide. Obviously, this also depends on the language you're building your tools in...I wrote a blog post to In The Box a few months back about parsing Webmin config files in several languages (perl, shell, PHP, Ruby and Python) here:
http://inthebox.webmin.com/one-config-file-to-rule-them-all