Ninja Forms user create

2.x.php

<?php
// Create user after form has been sanitised
add_action('ninja_forms_post_process', 'nf_create_user');
function nf_create_user(){
	
	// Call in Ninja Forms
	global $ninja_forms_processing;
	
	// Get the form ID
	$form_id = $ninja_forms_processing->get_form_ID();
	
	// If the form ID is the one we want - change the number here depending on form used
	if ( $form_id == 6 ){
	
		// Set up variables based on form values - change the numbers below based on field IDs
		$first_name = $ninja_forms_processing->get_field_value( 7 );
		$surname = $ninja_forms_processing->get_field_value( 8 );
		$u_email = $ninja_forms_processing->get_field_value( 9 );
		$username = $ninja_forms_processing->get_field_value( 10 );
		$password = $ninja_forms_processing->get_field_value( 11 );
			
		
		// Create an array of the user data - check out the available user fields for left hand side here: https://codex.wordpress.org/Function_Reference/wp_insert_user
		$userdata = array(
			'first_name'  =>  $first_name,
			'last_name'   =>  $surname,
			'user_login'  =>  $username,
			'user_email'  =>  $u_email,
			'user_pass'   =>  $password,
			'role'	      =>  'subscriber'
		);


		// Create the user and remember the user id
		$user_id = wp_insert_user( $userdata ) ;

		// Use the user id to log in straight away
		wp_set_auth_cookie( $user_id, true );
		
	}
	
}

3.x.php

<?php
// Create user after form has been sanitised
add_action( 'ninja_forms_after_submission', 'nf_create_new_user' );
function nf_create_new_user( $form_data ){
	$form_id = $form_data[ 'form_id' ];

	// If form ID is the one we want
	if( $form_id == 6 ){

		// Load up the fields we have for this form
		$form_fields = $form_data[ 'fields' ];

		// Set up variables based on form values - change the numbers below based on field IDs

		$first_name = $form_fields[ 7 ][ 'value' ];
		$surname = $form_fields[ 8 ][ 'value' ];
		$u_email = $form_fields[ 9 ][ 'value' ];
		$username = $form_fields[ 10 ][ 'value' ];
		$password = $form_fields[ 11 ][ 'value' ];


		// Create an array of the user data - check out the available user fields for left hand side here: https://codex.wordpress.org/Function_Reference/wp_insert_user
		$userdata = array(
			'first_name'  =>  $first_name,
			'last_name'   =>  $surname,
			'user_login'  =>  $username,
			'user_email'  =>  $u_email,
			'user_pass'   =>  $password,
			'role'	      =>  'subscriber'
		);


		// Create the user and remember the user id
		$user_id = wp_insert_user( $userdata ) ;

		// Use the user id to log in straight away
		wp_set_auth_cookie( $user_id, true );

 	}
}

It looks like line 5 of the 3.x snippet should check for 'form_id' instead of 'id'.

$form_id = $form_data[ 'form_id' ];

I also had to change how I was retrieving the form field values:

$first_name = $form_fields[ "7" ]["value"];