silverstripe-userforms/code/model/editableformfields/EditableMemberListField.php

<?php
/**
 * Creates an editable field that displays members in a given group
 *
 * @package userforms
 */

class EditableMemberListField extends EditableFormField {

	private static $singular_name = 'Member List Field';

	private static $plural_name = 'Member List Fields';

	private static $has_one = array(
		'Group' => 'Group'
	);

	/**
	 * @return FieldList
	 */
	public function getCMSFields() {
		$fields = parent::getCMSFields();

		$fields->removeByName('Default');
		$fields->removeByName('Validation');

		$fields->addFieldToTab(
			'Root.Main',
			DropdownField::create(
				"GroupID",
				_t('EditableFormField.GROUP', 'Group'),
				Group::get()->map()
			)->setEmptyString(' ')
		);

		return $fields;
	}

	public function getFormField() {
		if(empty($this->GroupID)) {
			return false;
		}

		$members = Member::map_in_groups($this->GroupID);
		$field = new DropdownField($this->Name, $this->EscapedTitle, $members);
		$this->doUpdateFormField($field);
		return $field;
	}

	public function getValueFromData($data) {
		if(isset($data[$this->Name])) {
			$memberID = $data[$this->Name];
			$member = Member::get()->byID($memberID);
			return $member ? $member->getName() : "";
		}

		return false;
	}
}