config.yml

# app/config/config.yml
twig:
    form:
        resources:
            - 'AcmeDemoBundle:Form:fields.html.twig'

ExampleType.php

<?php
// src/Acme/DemoBundle/Form/ExampleType.php
namespace Acme\DemoBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;

use Acme\DemoBundle\Form\Type\FieldsetType;

class ExampleType extends AbstractType {

    protected $options;

    public function __construct ( $options = array() ) {
        $this->options = $options;
    }

    public function buildForm ( FormBuilderInterface $builder, array $options ) {
        $builder
            ->add('firstname')
            ->add('lastname')
            ->add('email','email');

$builder->add('credentials',new FieldsetType(),array(
        'label' => false,
        'required' => false,
        'mapped' => false,
        'title' => 'Credentials',
        'subforms' => array(
            array(
                'name'=>'username',
                'type'=>'text',
                'attr'=> array(
                    'required' => true
                )
            ),
            array(
                'name'=>'password',
                'type'=>'password',
                'attr'=> array(
                    'required' => true
                )
            )
        )
    )
);
    }

    public function getName() {
        return 'acme_mainbundle_exampletype';
    }
}

fields.html.twig

{# src/Acme/DemoBundle/Resources/views/Form/fields.html.twig #}
{% block fieldset_widget %}
    {% spaceless %}
        <fieldset {{ block('widget_container_attributes') }}>
            {% if title is defined %}<legend>{{ title }}</legend>{% endif %}
            {{ form_widget(form) }}
        </fieldset>
    {% endspaceless %}
{% endblock %}

FieldsetType.php

<?php
// src/Acme/DemoBundle/Form/Type/FieldsetType.php
namespace Acme\DemoBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormView;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class FieldsetType extends AbstractType {

    public function setDefaultOptions ( OptionsResolverInterface $resolver ) {
        $resolver->setDefaults(array(
            'title'      => false,
            'subforms'   => array(),
            'options'    => array()
        ));
    }

    public function buildForm ( FormBuilderInterface $builder, array $options ) {
        if ( !empty($options['subforms']) ) {
            foreach ( $options['subforms'] as $f ) {
                $builder->add($f['name'],$f['type'],$f['attr']);
            }
        }
    }

    public function buildView ( FormView $view, FormInterface $form, array $options ) {
        if ( isset($options['title']) || $options['title'] !== false ) {
            $view->vars['title'] = $options['title'];
        }
    }

    public function getName() {
        return 'fieldset';
    }
}