Years ago when I first set up my Airport Extreme, I remember setting up a custom message that says "Welcome to the network". So every time users join the network they get that message in a pop-up. For the life of me I can't find how to remove it. Long ago when I added it, the Airport Utility UI was very different and I'm almost tempted to say that they didn't build into the new interface a way to add/remove this message. I've searched Apple forums and the internet in general. Nothing talks of a welcome message.