How can the sybil attack be used on a P2P system that implements DHT?

I was reading about the sybil attack and am unclear on some details.

After becoming part of the peer-to-peer network, the adversary may then overhear communications or act maliciously. By masquerading and presenting multiple identities, the adversary can control the network substantially.

To me some information was skipped over here. So with the sybil attack one entity (e.g. one person on one computer) can have multiple identities (e.g. look like they are several torrent clients) so what? How does this "control the network"?