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.

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.

Multisite User Management vs WPMU User own site