How can I set up a proxy / inturrupt web browsing from the wifi I am rebroadcasting.

I am using Connectify to rebroadcast wifi. Is there some way that I can set up a proxy so that when someone joins the network it shows a webpage with some info on it, or a way I can make it pop up in the middle of browsing? Sort of the way they do it in big airports to advertise the fact that you have to pay.