Is it possible to send message to shared iTunes library owners on the network? I wonder if this is a way (or an app) to send instant messages to iTunes library shared owners, visible on the network.

It would be really handy to be allowed to communicate with them :-)
I don't know about a specific app to send instant messages (I doubt one exists) BUT I like to rename my own shared library something akin to "if you don't stop stealing my internet I'll steal your credit card info".  Not that I actually know how to do that, but it usually scares them off my wireless.