If you were online when your guild got hacked and took a screenshot of the mess to follow, look at the start of all the quit messages and you should see another message saying that someone "joined your guild." The person listed there is most likely your hacker. Upload it as you would any other Maplenoobs screenshot (changing the filename from Maplexxxx.jpg to Hackersign.jpg in the process), then start a new page as you usually would, and use this template:
|image = (the filename of the image you just uploaded)
|guild = (the name of the affected guild)
|world = (the world that this occured on)
|status = (Active, or Off-Rank. Since this is recent it is unlikely that anyone not on the
rankings is banned.)
All reports are automatically added to the "Hackers" and "Guildhackers" category, the latter of which is accessed via the "Apr. '08 Guild Crisis" link under "search by offense."
An example of how one of these pages would look can be found here.
In the near future, I plan on compiling a list of as many guilds that have been hacked as part of a "memorial", so to say. This is quite a devastating event for the gMS community, and I feel lucky to have not lost my guild in the process. (Although we did have to resort to kicking everyone to preserve it.)