$fieldset->addField('group_aviable', 'text',
            array(
                'name'  => 'Group_Aviable',
                'label' => Mage::helper('customer')->__('Visible in register'),
                'title' => Mage::helper('customer')->__('Visible in register'),
                'note'  => Mage::helper('customer')->__('Visible in register'),
                'value' => '',
                'required' => false,
            )
        );

if (!is_null($customerGroup->getId()))