So now you have in place all the bits you need for a "members only" netnews feed, someone should go dig up the code and create a 'news' sync'd document. If you played your cards right you could use off the shelf news readers that look into a local repo of the news.
With just a tiny bit of crypto code you could add a repudiation feature (keys signed by the secret vs the secret itself) and control access to both individual groups and individual users.
Just to make the connection for others, you are describing something that can be used for chat AND binaries. As ugly a hack as it might be, (speaking to the choir, I know) there is tremendous value in simply being able to move messages (in this case, as simple files), between buddies without needing an "account" with a "service" (of the type that motivates people to excessively use "quotes").
With just a tiny bit of crypto code you could add a repudiation feature (keys signed by the secret vs the secret itself) and control access to both individual groups and individual users.