Customize the default user registration form

Firstly, in your theme's root directory add the following to template.php. If the file does not exist, create it.

function YOURTHEME_theme(&$existing, $type, $theme, $path){
  $hooks = array();
   // Make user-register.tpl.php available
  $hooks['user_register_form'] = array (
     'render element' => 'form',
     'path' => drupal_get_path('theme','YOURTHEME'),
     'template' => 'user-register',
     'preprocess functions' => array('YOURTHEME_preprocess_user_register_form'),
  );
  return $hooks;
}

function YOURTHEME_preprocess_user_register_form(&$vars) {
  $args = func_get_args();
  array_shift($args);
  $form_state['build_info']['args'] = $args; 
  $vars['form'] = drupal_build_form('user_register_form', $form_state['build_info']['args']);
}

Account fields

Account fields are rendered following the pattern of the two required fields, 'name' and 'mail':

print render($form['account']['name']);
print render($form['account']['mail']);

Profile2 Support

To load Profile2 fields into the user registration form manually, use this pattern:

print render($form['profile_theprofile_name']['field_thefield_name']);

Also, I make use of field group module (here) with Profile2 and noticed that I didn't need to include anything about the field group a Profile2 field is in to render that field.

CAPTCHA

If you use the CAPTCHA module and need to build your own user-register.tpl.php you'll need this to render it:

  print render($form['captcha']);

This took me longer than it should of to figure it out.

Submission

Lastly, if you want your form to actually be usable, the following snippet is required to build the form action stuff:

  print drupal_render($form['actions']);
  print drupal_render_children($form);

This page is a reference page spawned from this support thread in the Theme Development forum.