Author Access

The contributor cannot restrict access to a post to groups that he does not belong to.

Ah then this might not be a great feature request.

My use-case is a front-end application where the user can create and restrict posts. For documentation purposes, this is how you could allow post authors to always see their posts regardless of assigned Groups.

/**
 * Ensure the post author always has access to their published posts.
 *
 * This is not needed in a default implementation of Groups.
 *
 * @param String $sql
 * @param WP_Query $query
 *
 * @return String $sql
 */
function wpf15771076_groups_give_author_access( $sql, $query ) {

	global $wpdb;

	if( ! is_user_logged_in() ) {
		return $sql;
	}

	return sprintf( 'AND %s OR ( 1=1 %s )',
		$wpdb->prepare( "{$wpdb->posts}.post_author = %d", get_current_user_id() ),
		$sql
	);

}
add_filter( 'groups_post_access_posts_where', 'wpf15771076_groups_give_author_access', 10, 2 );