Reputation aggregation methods in unstructured Peer-to-Peer(P2P) networks are used to evaluate the trustworthiness of participating peers and to combat malicious peer's behaviors. In the reputation aggregation method, each peer calculates global scores by aggregating local scores which are obtained from each peer in P2P networks. In structured P2P networks, each peer exchanges local scores with other peers. Gossip Trust is proposed as a reputation aggregation method for the unstructured P2P networks. However, Gossip Trust has problem that high frequency of munications and long aggregation time increase as the number of peers in the network increase. In this paper, we propose a reputation aggregation method called "ID-List based Group Trust(ILGT)". ILGT limits the number of peers that icipate in reputation aggregation process and share the global scores by using list of ID. And ILGT improves communication frequency and aggregation time. Through computer simulations, we compare the proposed method ILGT with Gossip Trust.