I can suggest a couple of paths that you might find useful. The first is my my [answer][1] to a StackOverflow question, *Artificial Intelligence Methods to Detect Cheating in Games*. The question was not limited to collusion but that was in fact the scope of my answer. 

The second source is an excellent study by Andrew Odewahn, which i first learned in the O'Reilly compilation, *Beautiful Visualization* (ch 8). In this study Odewahn looks at congressional voting records for the purpose of of identifying *which members of congress tend to vote together* on the same bill. To do this, he creates an *Affinity Matrix* (which is analogous to an adjacency matrix, a common way to represent graph structures for computation.

Here is one of the rendered graphs from his study (rendered in graphviz) the two different colors represent the two political parties:
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![enter image description here][2]


  [1]: http://stackoverflow.com/questions/7492940/artificial-intelligence-methods-to-detect-cheating-in-games
  [2]: https://i.sstatic.net/H1Y4d.png