Modify user_register form to add custom form for adding to og_users_roles table.

rcg_register.info

name = RCG Register
description = Demonstrates how to alter the registration form with an AHAH element.
core = 6.x

rcg_register.module

<?php

/**
* Implement hook_menu().
*
* Note that each item has both an entry point to prepare the form, and also
* a callback function that provides and AHAH menu callback.
*/
function rcg_register_menu() {
  $items = array();

  $items['og_register_stuff'] = array(
    'page callback' => 'rcg_register_og_register_callback',
    'access callback' => TRUE,
    'type' => MENU_CALLBACK,
  );

  return $items;
}

function rcg_register_form_user_register_alter(&$form, &$form_state) {
  // This is how to add a section with a #ahah button. It's pretty much
  // identical to the stuff in the ahah example, but it is being added
  // with a form alter.
  $form['og_register']['ahahbutton'] = array(
    '#type' => 'submit',
    '#value' => t('Select Roles'),
    '#ahah' => array(
      'wrapper' => 'og-register-ahah-wrapper',
      'path' => 'og_register_stuff',
      'method' => 'append',
    ),
    '#validate' => array(),
    '#submit' => array(),
  );
  $form['og_register']['account_roles'] = array(
    '#prefix' => '<div id="og-register-ahah-wrapper">',
    '#suffix' => '</div>',
    '#type' => 'fieldset',
  );
  if (!empty($form_state['values']['op']) && $form_state['values']['op'] == t('Select Roles')) {
    $gid = $form_state['values']['og_register'];
    $form['og_register']['account_roles'][] = rcg_register_add_roles_form($gid);
  }

  // Add custom submit function to handle added roles form.
  $form['#submit'][] = 'rcg_register_og_roles_submit';
}

function rcg_register_og_register_callback() {
  $form = rcg_register_callback_helper();

  $fieldset = $form['og_register']['account_roles'];
  // Remove the wrapper so we don't double it up.
  unset($fieldset['#prefix'], $fieldset['#suffix']);

  $output = theme('status_messages');
  $output .= drupal_render($fieldset);

  // Final rendering callback.
  drupal_json(array('status' => TRUE, 'data' => $output));
  exit();
}

/**
* Does the very standard things that must be done in any normal callback.
* Used by each callback in this example module.
*/
function rcg_register_callback_helper() {
  $form_state = array('storage' => NULL, 'submitted' => FALSE);
  $form_build_id = $_POST['form_build_id'];
  $form = form_get_cache($form_build_id, $form_state);
  $args = $form['#parameters'];
  $form_id = array_shift($args);
  $form_state['post'] = $form['#post'] = $_POST;
  // Enable the submit/validate handlers to determine whether AHAH-submittted.
  $form_state['ahah_submission'] = TRUE;
  $form['#programmed'] = $form['#redirect'] = FALSE;
  drupal_process_form($form_id, $form, $form_state);
  $form = drupal_rebuild_form($form_id, $form_state, $args, $form_build_id);
  return $form;
}

/**
 * Form builder function for form to add roles.
 */
function rcg_register_add_roles_form($gid) {
  $account_node = node_load($gid);
  // Get roles that are defined for accounts in og.
  $og_account_roles = variable_get('og_user_roles_roles_account', '');
  $og_account_roles = array_filter($og_account_roles);
  foreach($og_account_roles as $rid) {
   $role_name = db_result(db_query("SELECT name FROM {role} WHERE rid = %d", $rid));
    $options[$rid] = $role_name;
  }

  $form['account_' . $gid] = array(
    '#type' => 'checkboxes',
    '#title' => t('Available roles for: ' . $account_node->title),
    '#options' => $options,
  );

  return $form;
}

/**
 * Custom submit function to submit og_roles form.
 */
function rcg_register_og_roles_submit($form, &$form_state) {
  foreach ($form_state['values'] as $key => $data) {
    if (substr($key, 0, 8) == 'account_') {
      $gid = substr($key, strpos($key, '_')+1);
      foreach (array_filter($data) as $rid) {
        og_user_roles_role_add($gid, $form_state['user']->uid, $rid);
      }
    }
  }
  $form_state['#redirect'] = 'user';
}