Redirect users from authentication URLs to signup page
There is no dedicated hook for things like this, but you can implement hook_init() and then do the redirect there. Something like this:

function yourmodule_init() {
  global $user;
  if (in_array($_GET['q'], array('user', 'user/login', 'user/register') &amp;&amp; !$user-&gt;uid) {
    drupal_goto('sign-up');
  }
}