Along the lines of AMQP, take a look at www.rabbitmq.com It's a doddle to set up, lightning quick (we regularly run at ~80,000 messages/sec between Europe and the US over the public internet), will run in RAM or on disk and supports quite a few message distribution models, including allowing an ad-hoc number of consumers to connect and receive messages - so each client would just register its own queue update consumer when it was fired up, and when an update is made in a client it could be published out to all.

It's worth doing the 6 tutorials so you can see just what it can do - a solution may present itself to you.