code: @Nonnull
    public boolean isServiceGroupRegistered (@Nonnull final IParticipantIdentifier aParticipantID)
    {
      ValueEnforcer.notNull (aParticipantID, "ParticipantID");
  
      final HttpGet aGet = new HttpGet (m_sPDIndexerURL + aParticipantID.getURIPercentEncoded ());
      try
      {
        return executeRequest (aGet, new PDClientResponseHandler ()).isSuccess ();
      }
      catch (final Throwable t)
      {
        m_aExceptionHdl.onException (aParticipantID, "isServiceGroupRegistered", t);
      }
      return false;
    }
docs: Gets a list of references to the CompleteServiceGroup's owned by the
  specified userId. This is a non-specification compliant method.
  
  @param aParticipantID
  Participant ID to query for existence. May not be <code>null</code>.
  @return <code>true</code> if the participant is in the index,
  <code>false</code> otherwise.