The script below uses API call hostgroup.massadd to add all hosts to "All" group. If run more than once, database table hosts_groups will have duplicate host-to-group membership entries (those been causing Zabbix performance to be really poor).

<?php
require_once("ZabbixAPI.class.php");

// This enables debugging, this is rather verbose but can help debug problems
#ZabbixAPI::debugEnabled(TRUE);

// This logs into Zabbix, and returns false if it fails
ZabbixAPI::login('http://localhost/zabbix/','apiuser','cut')
or die('Unable to login: '.print_r(ZabbixAPI::getLastError(),true));

1. Hardcoding group ID for group "All"
   $hostgroup[0] = "100100000000050";

1. Get all hosts
   $hosts = ZabbixAPI::fetch_column('host','get',array('extendoutput'=>1))
   or die('Unable to get hosts: '.print_r(ZabbixAPI::getLastError(),true));

1. Add all hosts to "All" group
   $result = ZabbixAPI::fetch_array('hostgroup','massadd',array('groups'=>$hostgroup,'hosts'=>$hosts))
   or die('Unable to get hosts: '.print_r(ZabbixAPI::getLastError(),true));
   #print "All group members: ".print_r($result, true)."\n<br>";
   ?>

After having this script running on a hourly basis I got this:

mysql> select count from groups;
----------

count

----------

----------
1 row in set (0.00 sec)

mysql> select count from hosts;
----------

count

----------

----------
1 row in set (0.00 sec)

mysql> select count from hosts_groups;
----------

count

----------

----------
1 row in set (0.00 sec)

mysql> select count from hosts_groups where groupid='100100000000050'; # <- All group
----------

count

----------

----------
1 row in set (0.08 sec)