The peerID is a random string assigned by the signalling server upon joining a game.
You can use PeerIDAt(index) to get the ID of each peer in order of joining.
The alias and ID of the nth peer in the current room, up to PeerCount.
If you are unfamiliar with PeerIDs, I highly recommend reading and going through the steps of making the Multiplayer Tutorials https://www.scirra.com/tutorials/892/mu ... 1-concepts. PeerID is a core concept and you won't get very far without understanding and working with them.