array_merg

$friends_ids = UM()->Activity_API()->api()->friends_ids();
            $friends_admin = array_merge($friends_ids, $admin_ids);
            if ( $friends_ids ) {
                $args['meta_query'][] = array(
                    'key'       => '_user_id',
                    'value'     => $friends_admin,
                    'compare'   => 'IN'
                );
            }