// the caller
$form = drupal_get_form('myform', $record_id);

// in the form builder...
function myform($form, &$form_state, $record_id) {
  // db query the table for that record id
  // populate form default values if record found

  // example
  $form['email']['#default_value'] = $query_result ? $query_result['email'] : NULL;
}