code: func (c *EtcdGroupService) GroupContainsMember(ctx context.Context, groupName, uid string) (bool, error) {
  	ctxT, cancel := context.WithTimeout(ctx, transactionTimeout)
  	defer cancel()
  	etcdRes, err := clientInstance.Txn(ctxT).
  		If(clientv3.Compare(clientv3.CreateRevision(groupKey(groupName)), ">", 0)).
  		Then(clientv3.OpGet(memberKey(groupName, uid), clientv3.WithCountOnly())).
  		Commit()
  
  	if err != nil {
  		return false, err
  	}
  	if !etcdRes.Succeeded {
  		return false, constants.ErrGroupNotFound
  	}
  	return etcdRes.Responses[0].GetResponseRange().GetCount() > 0, nil
  }
docs: // GroupContainsMember checks whether a UID is contained in current group or not