Packt publishing have just released a new book on Drupal 6, titled 'Drupal 6 Social Networking'. They sent me the following article:
Drupal 6 Social Networking: Communicating with Users
Our users can communicate with one another, which is great, but quite often as an administrator the need may arise for us to communicate with a user or users on our site. It may be to remind them about the web site or to inform active users about new changes to the site, which they may not have been made aware of.