List of customers in customer groups

Question

I have around 80 different customer groups, with different prices on.

How can I find out how many customers are in any given group?

Can I make a list of all customers included in customer group?

I have this but it gives me error Fatal error: Class 'Mage' not found in /domains/dev.domain.co.uk/http/customer_groups.php on line 3

UPDATE: adding:

require_once('app/Mage.php');
Mage::app();

Fixed it.

Update 2:

Error: Fatal error: Call to a member function addField() on a non-object in /domains/dev.domain.co.uk/http/customer_groups.php on line 19

Where do I have to place the code to get the result. I found this code and this supposed to be a outcome but I'm struggling implementing it.

UPDATE 3

This is my code, it displays almost everything exactly as I want, except, there are gaps in between the groups, also could I have a group ID with it too?

require_once('app/Mage.php');
Mage::app();


$_customer = array();
$customerCollection = Mage::getResourceModel('customer/customer_collection');
foreach ($customerCollection as $customer) {
    $_customer[$customer->getGroupId()][] = $customer->getId();
}

$_groups = array();
$groupCollection = Mage::getResourceModel('customer/group_collection');
foreach ($groupCollection as $group) {
    $_groups[$group->getId()] = $group->getCode();
}

foreach ($_customer as $groupId => $customerIds) {
    echo 'Group "' . $_groups[$groupId] . '" has ' . count($customerIds) . ' customers.<br>';
}

Answer 1

You have to initiate Mage first. Add to top of your file ...

require_once('app/Mage.php');
Mage::app();

Edit

Please try this ...

$_customer = array();
$customerCollection = Mage::getResourceModel('customer/customer_collection');
foreach ($customerCollection as $customer) {
    $_customer[$customer->getGroupId()][] = $customer->getId();
}

$_groups = array();
$groupCollection = Mage::getResourceModel('customer/group_collection');
foreach ($groupCollection as $group) {
    $_groups[$group->getId()] = $group->getCode();
}

foreach ($_customer as $groupId => $customerIds) {
    echo 'Group "' . $_groups[$groupId] . '" has ' . count($customerIds) . ' customers.<br>';
}

^{Maybe not best-practice. This can be done in one collection using joinLeft on customer group table ...}