silverstripe-userforms/code/formfields/UserFormsGroupField.php

<?php

/**
 * Front end composite field for userforms
 */
class UserFormsGroupField extends UserFormsCompositeField {

	public function __construct($children = null) {
		parent::__construct($children);
		$this->setTag('fieldset');
	}

	public function getLegend() {
		// Legend defaults to title
		return parent::getLegend() ?: $this->Title();
	}

	public function processNext(EditableFormField $field) {
		// When ending a group, jump up one level
		if($field instanceof EditableFieldGroupEnd) {
			return $this->getParent();
		}

		// Otherwise behave as per normal composite field
		return parent::processNext($field);
	}
}