You can follow following,
1. use fep_form_field_output_fep_pro_to action hook to create a checkbox in “To” field
2. use 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 fep_errors()->remove('pro_to') in fep_action_validate_form hook

Note: I will try to add Group messaging feature in next major version.