Not sure what you mean by a sub-session but my perusal suggests an entirely new conversation key is negotiated by current participants when those participants change. The spec doesn't say anything about requiring everyone to be online but I think it's implied. It may be that not everyone has to be online at the same time (which would just delay the negotiation IIUC) which is interesting but I wonder what would happen if an offline participate rejoins and doesn't get a full transcript from when they were last online with the carrier. Sounds entirely possible for an IRC/XMPP carrier with people not using bouncers.