Group contacts based on the number of times they have completed an activity

[wwatson13]

I am working on implementing CiviCRM for a wellness center, where they group their clients in different levels based on the number of their visits. So 25 visits is green, 50 visits purple, etc.  So the implementation is that each visit will be an activity, wellness visit,  and what I need to be able to do is identify all contacts between 25 and 49 visits to the green group, etc.

I am running civicrm on top of Drupal so I was thinking I would tie this behavior to rules to notify contacts when their level changes.

Any ideas on how this could be accomplished?