Amendment of https://gist.github.com/Apina/5576163 to change the user role on specific event ID.

custom_functions.php

<?php //Please do not include the opening PHP tag

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( $user_id );
		if($attendee_data['event_id'] == *EVENTIDHERE*)
    		{
			$user->set_role( 'contributor' );
		} else {
		      $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
}