module's local *.tpl.php, defined in $theme_registry, "not found or invalid" when referenced later in module
drupal_get_form() expects the name of a function that builds a form array, e.g.

function MYMODULE_some_form($form, &amp;$form_state) {
  $form['#theme'] = 'user_login_member_form';

  $form['foo'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('Bar'),
    '#default_value' =&gt; t('Baz'),
  );

  $form['submit'] = array(
    '#type' =&gt; 'submit',
    '#value' =&gt; t('Submit'),
  );

  return $form;
}

function MYMODULE_some_form_submit($form, &amp;$form_state) {
  // Process $form_state['values']
}

// In a hook...
$form = drupal_get_form('MYMODULE_some_form');


You don't need to add the form function to a registry as such, just giving it the same name as the string you pass to drupal_get_form() is enough.

hook_theme() is for registering theme functions, which is why you're currently getting an error.

The Form generation docs have a bit more information, and the FAPI reference is invaluable if you're new to the form API. For theming the form, there are some excellent answers to this question which should help.