This action creates a WordPress user when someone registers for your an event. It uses the email address of the registrant to create the user name and the password is randomly generated. Add this function and action to your theme/functions.php file. More info in the comments below.

espresso_create_wp_user.php

<?php
add_action('action_hook_espresso_save_attendee_data','espresso_create_wp_user', 10, 1);
function espresso_create_wp_user($attendee_data) {
    if( username_exists( $attendee_data['email'] ) == NULL ) {
		global $org_options;	
		
		// Generate the password and create the user
		$password = wp_generate_password( 12, false );
		$user_id = wp_create_user( $attendee_data['email'], $password, $attendee_data['email'] );
	
		// Set the users details
		//Additional fields can be found here: http://codex.wordpress.org/Function_Reference/wp_update_user
		wp_update_user(
			array(
				'ID'			=> $user_id,
				'nickname'		=> $attendee_data['fname'] . ' ' . $attendee_data['lname'],
				'display_name'	=> $attendee_data['fname'] . ' ' . $attendee_data['lname'],
				'first_name'	=> $attendee_data['fname'],
				'last_name'		=> $attendee_data['lname'],
				'description'	=> __('Registered via event registration form.', 'event_espresso'),
			)
		);
	
		// Set the role
		$user = new WP_User( $user_id );
		$user->set_role( 'subscriber' );
	
		// Email the user
		wp_mail( $attendee_data['email'], 'Welcome to ' . $org_options['organization'], 'Your Username: ' .$attendee_data['email']. ' Your Password: ' . $password );
	
	} // end if
}

Hi Fellow event exspresso developers is there a update for this function in EE4 or does anybody know of any hooks that are available for to add this code to in EE4