Have users created automatically when a network site is created

You can do that hooking into wpmu_new_blog and then creating the user if it does not exists, and adding that user to the blog being created.
To do so, you will use a code like this ( only for 1 user) , as a network enabled plugin or on the main theme functions.php

add_filter('wpmu_new_blog','wpse_29543_new_blog');

function wpse_29543_new_blog($blogid){
    $user_name="user";
    $user_password="password";
    $user_email="[email protected]";
    $role="editor";
    $user_id = username_exists( $user_name );
    if ( !$user_id ) {  
        $user_id = wp_create_user( $user_name, $user_password, $user_email );
    } 
    add_user_to_blog( $blogid, $user_id, $role );

}