Multisite Registration Form?

For add form for multisite network you need to create form using WP User Form Builder Addon (Pro).
For every site you need to create different forms.

WP User Form Builder Features :

Create multiple front-end registration forms (Show different fields for each group)
Assign a specific User Role automatically for each registration form
Customizable Front-end Registration
Customizable Front-end User Profiles
Unlimited custom fields plus many pre-defined fields
You can create multiple registration forms for your site so each user role has a unique registration form

Please check following link for more information :
https://wpuserplus.com/blog/doc/wp-user-form-builder/

You need to buy pro version for custom fields.
https://wpuserplus.com