E-mail reveals real leaders [via Column Two]: how Hewlett-Packard discovers communities of practice by analysing intenal e-mail exchange. Contains reference to the paper, which describes the algorithm in more details:
Tyler, J. R., Wilkinson, D. M. & Huberman, B. A. Email as spectroscopy: automated discovery of community structure within organizations. Preprint http://xxx.lanl.gov/abs/cond-mat/0303264, (2003).
Looks like an interesting idea. Something to get back to and read in detail