D7 Node Field: Create OG Member Checklist in Node Edit Form
Use hook_form_alter to do a entity field query or SQL query to get a list of UIDs that belong to 1 group ... OG may have a function to retrieve that list I dunno.

Once you have a list of UIDs use Devel and dpm($form); the form object and you can simply unset() the #options values for UIDs that dont belong in the OG group. Here's a similar thing I did for limiting to a taxonomy term_reference field to a given level of a nested taxonomy.

If you're paranoid you can also append your own validator function to the form_submit function and cross check that valid UIDs (members in the OG group) are being saved .... but I dont usually bother to do that for intranet sites (I have "trusted" users / staff members ... not joe blow public).

Sample code doing basically the same thing for a taxonomy term reference field.

/**
 * Implements hook_form_alter().
 */
function eap_student_videos_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    switch ($form_id) {
      // Field specific overrides.
      case 'eap_studentvideos_node_form':
          $form['field_eap_video_country']['#after_build'][] = 'eap_country_term_limit_to_countries';
          break;
    }
}

function _eap_region_tids() {
  return array(24,35,48,8,4);
}


function eap_country_term_limit_to_countries(&amp;$element) {
    $options = $element[LANGUAGE_NONE]['#options'];
    $result = db_query('
        SELECT tth.tid, ttd.name AS term_name
        FROM taxonomy_term_hierarchy tth
        LEFT JOIN taxonomy_term_data ttd ON tth.tid = ttd.tid
        WHERE tth.parent IN  (:tids)
        ORDER BY 2 ASC
    ', array(':tids' =&gt; _eap_region_tids()));
    $country_tids = $result-&gt;fetchAllAssoc('tid');
    $country_tid_array = array_combine(array_keys($country_tids), array_keys($country_tids));
    $new_options = array_intersect_key($options, $country_tid_array);
    array_walk($new_options, function(&amp;$value, &amp;$key) { $value = str_replace('-','',$value); });
    // array_merge() destroys the array keys.
    $new_options = array('_none' =&gt; '- Select a value -') + $new_options;
    $element[LANGUAGE_NONE]['#options'] = $new_options;
    return $element;
}