If two clients each called A and B wish to P2P communicate then they need to be members of at least one of the same P2P group.
In typical internet messenger service :
P2P Group of ProudNet is also in the same concept. In internet messenger, each chatting windows respond to their associated P2P groups. Only difference is the authority of creating chatting window or entering a chatting window is given to server.
Identifier of P2P group is also Proud.HostID type.