Custom module embedding existing node form
node_form() is in node.pages.inc which isn't included by default. Just add a call like this to the beginning of your my_signup() function:

module_load_include('inc', 'node', 'node.pages');


When you call drupal_get_form() with a specific type (like signup_node_form) the node module internally translates that to node_form(). This happens in node_forms() which is in the node.module file, and as such is always included. 

However node.pages.inc is only included when specifically requested by a module/menu callback etc. so to use node_form() you'll need to include the file manually.

You also need to pass a node object to the function with the required content type assigned like the following code:

$new_node = new stdClass;
$new_node-&gt;type = 'signup';
$new_node-&gt;language = LANGUAGE_NONE;
$form = drupal_get_form('signup_node_form', $new_node);