About two days, I have search for some materials that can support me for finding clique based on the activities and relationship between users from email inbox and sent box.
Here is some links that I hope can help to give me undernstanding about finding clique algorithm from a graph:
The Algorihtm Design Manual
Clique (a short description from Wikipedia)
An Algorithm for Finding a Maximum Clique in a Graph
Finding a maxium clique in an arbitrary graph
The idea of this work is based on EMT Research from University of Columbia. This research is very great for determine viral propagation based on user behaviour. You can visit and take a look for more detail at http://www1.cs.columbia.edu/ids/emt/.