There are two communication phases for a Copay wallet: The wallet creation phase, which is when the copayers do not yet know the public keys of the other copayers. Then the wallet usage phase, where the copayers do know the public keys of the other copayers.
Each copayer independently generates a master extended public/private key pair before wallet creation which can be used to derive further public/private keys deterministically.
The author of the wallet derives a new public/private key pair for the communication purposes. This is the communication public/private key pair of the author.
The hash of the communication public key is used as the PeerJS id.