You would still need some form of STUN server (but there are a number of 'freely accessible' ones, even configured as defaults in some browsers) to get your reachable address/port/proto. You cant, AFAIK, handle this manually - it is handled internally as part of ICE. Then you would need to manually handle the signalling of these as well.
Thats the bare minimum you have to do to peer over webrtc under ideal circumstances, but its certainly doable.
So you do that for each peer. If you (or a peer) change streams you will do the same thing all over again