Usernames as subdomains and a root website

[eluser]BrianL[/eluser]
Subdomains are handled by the web server, not CI.

If you're using cPanel handle subdomain redirection through subsites, not subdomain, as the cPanel meaning of subdomain is different than the DNS meaning of subdomain. If not, hosting two totally different websites with the same IP is called virtual hosting and the method is specific to your webserver. Apache with Debian is particularly elegant, but all major webservers support virtual hosting.