A custom Drupal module using the emfluence Marketing Platform api php helper library along with webform based forms.

readme.md

Start by cloning the latest version of the api helper from https://github.com/jacobischwartz/emfl-api_php-lib to sites/all/libraries/emfl_api.

After that you'll create a custom module in sites/all/modules.

emfluence-marketing-platform.module

<?php

/**
 * Definitions
 */

define('EMFLUENCE_PLATFORM_API_KEY', 'ff342fde-a672-9fbb-d5aa-0f87487f1682');
define('EMFLUENCE_PLATFORM_ALL_USERS_LIST_ID', 0000);
define('EMFLUENCE_PLATFORM_SUBSCRIBERS_LIST_ID', 000000);

// The constants below are not the only place this number is used
// Be sure to update hook_{#} function as well
define('CUSTOM_WEBFORM_NID', 1);

/**
 * Includes
 */
require_once('webforms/webforms.forms.inc');

/**
 * Implementation of hook_{form_id}_alter
   @see CUSTOM_WEBFORM_NID
 * @param array $form
 * @param array $form_state
 */
function _custom_form_webform_client_form_1_alter(&$form, &$form_state){
  module_load_include('inc', 'custom', 'webform/webform.forms');
  _custom_form_webform_client_form_1_alter($form, $form_state);
}

webforms.forms.inc

/**
 * Implementation of hook_form_alter
 */
function _custom_form_webform_client_form_1_alter(&$form, &$form_state) {
  $form['#validate'][] = '_custom_form_webform_client_form_1_alter_validate';
  $form['#submit'][] = '_custom_form_webform_client_form_1_alter_submit';
}

/**
 * Implementation of hook_form_validate
 */
function _custom_form_webform_client_form_1_alter_validate(&$form, &$form_state) {
  $mailme_key = 'subscribe_to_newsletter';
  $vals = &$form_state['values']['submitted'];
  $mailme = ($vals[$mailme_key][$mailme_key] === $mailme_key);
  if ($mailme) {
    $now_required = array(
      'country',
      'address_1',
      'city',
      'state',
      'postal_code'
    );
    foreach ($now_required as $k) {
      if (empty($vals['mailing_address'][$k])) {
        form_set_error("submitted][mailing_address][$k", t('This field is required to be mailed a newslsetter.'));
      }
    }
  }
}

/**
 * Implementation of hook_form_submit
 */
function _custom_form_webform_client_form_1_alter_submit(&$form, &$form_state) {
  // Get form value function
  $gfv = function($key, &$fsv) {
    return !empty($fsv[$key]) ? $fsv[$key] : '';
  };

  // Form submitted values
  $fsv = &$form_state['values']['submitted_tree'];

  $data = array();
  $data['groupIDs'] = array(EMFLUENCE_PLATFORM_SUBSCRIBERS_LIST_ID);
  $data['originalSource'] = 'example.com/connect';
  $data['action'] = 'add';
  $data['firstName'] = $gfv('first_name', $fsv);
  $data['lastName'] = $gfv('last_name', $fsv);
  $data['email'] = $gfv('email_address', $fsv);

  $mailme_key = 'subscribe_to_newsletter';
  $mailme = false;
  if (isset($fsv[$mailme_key])) {
    if (isset($fsv[$mailme_key][0])) {
      $mailme = ($fsv[$mailme_key][0] === $mailme_key);
    }
  }

  if ($mailme) {
    $data['groupIDs'][] = EMFLUENCE_PLATFORM_SUBSCRIBERS_LIST_ID;
    $map = array(
      'country' => 'country',
      'address_1' => 'address1',
      'address_2' => 'address2',
      'city' => 'city',
      'state' => 'state',
      'postal_code' => 'zip',
    );
    foreach ($map as $f => $d) {
      $data[$d] = $fsv['mailing_address'][$f];
    }
  }

  require_once( 'sites/all/libraries/emfl_api/api.class.inc' );
  $platform_api = new Emfl_Platform_API( EMFLUENCE_PLATFORM_API_KEY );
  $platform_api->contacts_save($data);
}