Invite: unicast message with group identification

Group metadata: download from the swarm

Message multicast ideas:

1) Bittorrent

Swarm downloads: when to update? Polling.

2) Bitcoin

How are transfers propagated?

3) Fossil

Create a new ID when a new message is created, propagate that, and use
swarm to download the content. When messages history diverge, use
merging. Try to solve a problem regarding message ordering.