Co-Authors Plus - add custom "guest-author" fields

core.php

<?php

add_filter( 'coauthors_guest_author_fields', 'add_guest_author_fields', 10, 2 );

/**
 * Adds the custom guest author fields to the coauthors guest author fields array.
 *
 * @param array $fields_to_return The fields to return.
 * @param array $groups The groups to return.
 * @return array The fields to return.
 */
function add_guest_author_fields( $fields_to_return, $groups ) {

	$global_fields = array();

	$socials = array(
		__( 'Twitter', 'my-text-domain' ),
		__( 'LinkedIn', 'my-text-domain' ),
	);

	foreach ( $socials as $social ) {
		$global_fields[] = array(
			'key'   => sanitize_key( $social ),
			'label' => $social,
			'group' => 'contact-info',
			'input' => 'url',
		);
	}

	foreach ( $global_fields as $single_field ) {
		if ( in_array( $single_field['group'], $groups ) || 'all' === $groups[0] && 'hidden' !== $single_field['group'] ) {
			$fields_to_return[] = $single_field;
		}
	}

	return $fields_to_return;
}