Hi SPABO,
if you are using v4 then you will have to do it the way Bob has posted, and you will need a custom code action to format the results (Bob already posted this too) before you feed them to the "Dynamic data" in the dropdown settings.
However, if you are going to use v5 then things will be much easier, first, use this code instead since you will need the list under the model:
<?php
$list = \GCore\Admin\Models\User::getInstance()->find('all', array('fields' => array('id', 'name'), 'conditions' => array('GroupUser.group_id' => 2)));
$form->data["User"] = \GCore\Libs\Arr::getVal($list, array("[n]", "User"), array());
Then in your "Dynamic data" for the dropdown, enter "User" in the "Data path" and id for the "value" and "name" for the "text".
Now, when the form submits you need to find the selected user's email and inject it in the form data array, use this code in a custom code action before the email action:
<?php
$form->data["dropdown_name"] = \GCore\Admin\Models\User::getInstance()->field("email", array("id" => $form->data["dropdown_name"]));
Finally use "dropdown_name" (which is your dropdown's field name) in the "Dynamic to" of the Email action!
Regards,
Max