Re: Map BP Groups to Subdomains
http://keyboarddance.wordpress.com/2007/09/28/dynamic-subdomains-using-htaccess/ suggests the following .htaccess addition in the (given that wildcard DNS is enabled):
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www.yourwebsite.com
RewriteCond %{HTTP_HOST} ([^.]+).yourwebsite.com
RewriteRule ^(.*)$ /path_to_your_site/httpdocs/work_out.php?url=%1
Is that all it takes?