Geweb

Use this plugin if you want allow users create their own sites based on your.
<h3>Arbitrary section</h3>
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.

WPMU User own site

Plugin allows to create site on user registration in WPMU environment.

thenbrent

Running a WordPress network? You no longer need to manually add new users to each of your sites.
With this plugin, users are assigned a default role for each of your sites. You set the default role for each site and this plugin applies it.
You can assign different roles for each site or keep a site private by assigning no role.

Multisite User Management

Automatically add users to each site in your WordPress network.

WPMU User own site vs Multisite User Management