You must edit file /components/com_user/models/registration.php line 549
$emailBodyAdmin = JText::sprintf(
'COM_USERS_EMAIL_REGISTERED_NOTIFICATION_TO_ADMIN_BODY',
$data['name'],
$data['username'],
$data['siteurl']
);
The template is on /languages/your-language/your-language.com_users.ini (ex: /languages/en-GB/en-GB.com_users.ini) seek for COM_USERS_EMAIL_REGISTERED_NOTIFICATION_TO_ADMIN_BODY define
COM_USERS_EMAIL_REGISTERED_NOTIFICATION_TO_ADMIN_BODY="Hello administrator, \n\nA new user '%s', username '%s', has registered at %s."
first $s is $data['name'], second $s is $data['username'], etc.
So, if you want to add password:
$emailBodyAdmin = JText::sprintf(
'COM_USERS_EMAIL_REGISTERED_NOTIFICATION_TO_ADMIN_BODY',
$data['name'],
$data['username'],
$data['siteurl'],
$data['password_clear']
}
Then
COM_USERS_EMAIL_REGISTERED_NOTIFICATION_TO_ADMIN_BODY="Hello administrator, \n\nA new user '%s', username '%s', has registered at %s with password:%s."
You can also add all $data contents
foreach($data as $key=>$value) {
$emailBodyAdmin .= $key." = ".$value."\n";
}
Or write custom $emailBodyAdmin field by field.