Add Twitter handle/username to User Contact Information

add_user_twitter.php

<?php
/**
 * Add Twitter handle/username to User Contact Information
 *
 * @param $user_contact
 *
 * @return array
 */
function user_contact_add_twitter( $user_contact ) {
	$user_contact['twitter'] = __( 'Twitter Username' );

	return $user_contact;
}
add_filter( 'user_contactmethods', 'user_contact_add_twitter' );

get_twitter_user.php

<?php
/**
 * Get Twitter username for current post author
 */
$twitter = get_the_author_meta( 'twitter', get_the_author_meta( 'ID' ) );