For using Groundhog you will need to install 2 components:
Sketchup (2016 or earlier)
XQuartz (Only for OSX users that use version El Capitan or later)
While Groundhog is absolutely free, SketchUp may or may not be free, depending on what you want to do with it.
Trimble SketchUp is a friendly 3D modeling tool that is distributed in two versions:
SketchUp make: A free version of SketchUp for non-commercial and educational use.
SketchUp pro: The commercial version of SketchUp, that has some more features than the "make" version.
Groundhog can be used with either of those versions, which you can get from SketchUp.com.
Weather you choose SketchUp pro or make is up to you. Groundhog will not be responsible for any agreement you make with Trimble.
Groundhog is the extension you install over SketchUp. You may download it from groundhogproject.org.
Notice that you will have to choose the appropriate version for you, depending on your Operative System.
The Radiance binaries that come with Groundhog are a courtesy of the National Renewable Energy Laboratory (NREL)
An article of this exists in the SketchUp documentation. But, since we are not sure how long it will be kept there, we will provide some instructions:
Go to "Preferences" (in the Window menu, in Windows; or in SketchUp menu in Mac)
Select the "Extensions" pane
Click on "Install extension"
An open dialog will appear. Choose the "Groundhog.rbz" file you downloaded.
They may warn you about installing 3rd party extensions... just agree if you trust us.
This is relevant only for those who use Mac OS X
From "El Capitan" and afterwards, OSX does not come with XQuartz installed. This utility is required to run some Radiance functions, like previewing the model.
Please download and install XQuartz from its website https://www.xquartz.org/