Step 1: Install the Guild Info Table mod to World of Warcraft. Go to your World of Warcraft directory (probably C:\Program Files\World of Warcraft) and go to the subdirectories Interface\AddOns. Create a new directory there called GuildInfoTable, and download the three files below to that directory.
The next time you run World of Warcraft and bring up the Guild frame ("O"), you should see a message in your chat log saying "GuildInfoTable: Exported 48 guild members" (depending on how many guild members there are!) There should also be a small save button in the upper left corner of the frame - if for some reason it doesn't save the data automatically, you can click the button and it will save it.
Step 2: Download the Guild Data Sender. Download the file below to any directory (probably somewhere in Program Files) and unzip it:
Step 3: Installing the Guild Data Sender. Open a command prompt (Start->Run and type cmd) and navigate to that directory. Run guildDataSenderWin.exe -install - this will install the Guild Data Sender as a Windows service.
Step 4: Starting and configuring the Guild Data Sender. Go to the Control Panel, and go to the Administrative Tools and double-click on Services. In the list of services, find the Gwydden Telaid GuildInfoTable Service. Right-click on it and select Start. Then, right-click on it again and select Properties. Under the General tab, change the Startup Type to Automatic. This will make the service start at boot time.
That's it! The service will transmit the files every twenty minutes, so the info page will never be too far out of date as long as you remember to bring up the guild frame every now and then.
If you have any problems, post to the forums or drop me an email.
Source for the sender and receiver (the parser code is on the info page):
If you're running on a non-Windows platform, you can use the sendGuildData.py script to send in the data manually. Just change the WoW directory and the server to send it to.