Write custom metadata

  groups_update_groupmeta($GroupId, 'MyValueName', $MyValue);

Read custom metadata

  $MyValue = groups_get_groupmeta($GroupId, 'MyValueName');
  if ($MyValue == "")    //Will be "" if not stored
    $MyValue = "abc";

Delete custom metadata

  groups_delete_groupmeta($GroupId, 'MyValueName');