Filter posts SQL query to include additional authors.

additional-authors-partial.php

<?php

/**
	 * Hooks
	 *
	 * @return void
	 */
	public function init() {
		// Filter author query to include posts author was added to as an additional author.
		add_filter( 'posts_where', [ $this, 'filter_where' ], 10, 2 );
		add_filter( 'posts_join', [ $this, 'filter_join' ], 10, 2 );
		add_filter( 'posts_groupby', [ $this, 'filter_groupby' ], 10, 2 );
	}

	/**
	 * Filters the WHERE clause of the author query.
	 * This is needed because a standard WP meta_query always uses AND instead of OR.
	 *
	 * @see WP_Meta_Query::get_sql_clauses()
	 *
	 * @param string   $where The WHERE clause of the query.
	 * @param WP_Query $query The WP_Query instance (passed by reference).
	 *
	 * @return string
	 */
	public function filter_where( $where, $query ) {

		if ( ! is_admin() && $query->is_main_query() && $query->is_author() ) {
			global $wpdb;
			$author_id = $query->get_queried_object_id();
			// Add meta query for author ID pattern.
			$where .= $wpdb->prepare( " OR $wpdb->postmeta.meta_key = 'additional_authors' AND $wpdb->postmeta.meta_value LIKE %s AND $wpdb->posts.post_status = 'publish'", "%d:$author_id;%" );
		}

		return $where;
	}

	/**
	 * Filters the JOIN clause of the author query.
	 *
	 * @param string   $join  The JOIN clause of the query.
	 * @param WP_Query $query The WP_Query instance (passed by reference).
	 *
	 * @return string
	 */
	public function filter_join( $join, $query ) {

		if ( ! is_admin() && $query->is_main_query() && $query->is_author() ) {
			global $wpdb;
			// Add the INNER JOIN required for the meta query.
			$join = "INNER JOIN $wpdb->postmeta ON ( $wpdb->posts.ID = $wpdb->postmeta.post_id )";
		}

		return $join;
	}

	/**
	 * Filters the GROUP BY clause of the author query.
	 *
	 * @param string   $groupby The GROUP BY clause of the query.
	 * @param WP_Query $query   The WP_Query instance.
	 *
	 * @return string
	 */
	public function filter_groupby( $groupby, $query ) {

		if ( ! is_admin() && $query->is_main_query() && $query->is_author() ) {
			global $wpdb;
			// Add the needed GROUPBY clause to combine duplicate results from the meta query using a JOIN.
			$groupby = "$wpdb->posts.ID";
		}

		return $groupby;
	}