code: public void getMemberCount(AVIMConversationMemberCountCallback callback) {
      if (StringUtil.isEmpty(getConversationId())) {
        if (null != callback) {
          callback.internalDone(new AVException(AVException.INVALID_QUERY, "ConversationId is empty"));
        } else {
          LOGGER.w("ConversationId is empty");
        }
        return;
      }
      InternalConfiguration.getOperationTube().processMembers(client.getClientId(), conversationId, getType(), null,
              Conversation.AVIMOperation.CONVERSATION_MEMBER_COUNT_QUERY, callback);
    }
docs: 查询成员数量
  @param callback