You can use this module with the following in your ~/.xmonad/xmonad.hs:

 import XMonad.Layout.Grid

Then edit your layoutHook by adding the Grid layout:

 myLayouts = Grid ||| Full ||| etc..
 main = xmonad defaultConfig { layoutHook = myLayouts }

You can also specify an aspect ratio for Grid to strive for with the GridRatio constructor. For example, if you want Grid to try to make a grid four windows wide and three windows tall, you could use

 myLayouts = GridRatio (4/3) ||| etc.

For more detailed instructions on editing the layoutHook see:

XMonad.Doc.Extending