Programmatically load and submit the user form
You can try to prepoulate value of any form programmatically &amp; submit like this

$form_state = array();
$form_id = 'invite_form';
$form_state['values']['email'] = 'abc@xyz.com';
drupal_form_submit($form_id, $form_state);

If you want to gather data during user save as mention in question use hook_user_presave.