Copying from the widgets settings I made something that sort of works:
Define the setting to be type mixed:
protected function defineSettings()
{
return array(
'jobsemail' => array(AttributeType::Mixed, 'required'=>false),
);
}
Then create a settings.html like this:
{% import "_includes/forms" as forms %}
{% set userInput %}
<div class="select">
<select id="jobsemail" name="jobsemail">
<option value="">No one</option>
{% for user in craft.users.group('Admin') %}
<option value="{{ user.id }}"{% if user.id == settings.jobsemail %} selected{% endif %}>{{ user.name|t }}</option>
{% endfor %}
</select>
</div>
{% endset %}
{{ forms.field({
label: "Email recipient"|t,
instructions: "This person gets emailed when new jobs are added"|t,
id: 'jobsemail',
}, userInput) }}
This gives you a basic SELECT drop-down list. It's ok here, because the number of admin users is low. It would be less suitable if there were 100s of users.
Is a better solution with a real pop-up element picker would be nice!