You can follow following,
fep_form_field_output_fep_pro_to action hook to create a checkbox in “To” field
fep_form_field_validate_fep_pro_to to check if your checkbox is checked, if checked than you can query your group members on the fly and added in
$_POST['message_to_id'] as array.
3. You may need to use
Note: I will try to add Group messaging feature in next major version.