Is there an application or a site grouping addresses by user graphically?
The tool is called Blockviewer.  The website, blockviewer.com, no longer exists.  However, the source code is available on Github: https://github.com/thallium205/BitcoinVisualizer.  Thus, you might be able to download the source code, compile it, and run it on your own machine.

You can also read the original blog post introducing some of the ideas behind it.