The way I see it, is there are two approaches to group messaging that are being discussed:

The first is cryptographically secured group messaging, using decryption links.

This is not implemented, but would be cool if it was. Does anyone know how to do this? Dominic said 'coming soon', so I assumed he had done some work on it.

The second is protocol-enforced group messaging, where you expect scuttlebot to only gossip with your friends.

I think what @aljoscha is saying is that trying to get protocol-enforced group messaging to work -- without encrypting the messages -- is impossible. The messages will just leak, because people will modify their scuttlebots to get messages from beyond their foaf range.

You gotta either encrypt the messages for specific people, or share a decryption key for a specific thread of messages -- otherwise the messages will just leak to whoever wants to read and append to them.

You can enforce not replicating a feed, but you can't enforce not leaking an unencrypted feed.

The only way to create real secret group messaging is with cryptography.

And then you gotta remember that all crypto will eventually be broken, so don't send anything that you don't want to leak in the future.