WPMU User own site
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Plugin allows to create site on user registration in WPMU environment.
Use this plugin if you want allow users create their own sites based on your.
Arbitrary section
You can create your own HTML code of the form. Create file in your theme, put code from plugins/wp-auto-mu/geweb-mu/register.php to your new file and add_filter geweb-mu-register-path to rewrite file paht for the form.
This plugin is available for download for your site.