Custom Drupal Analytics with Views- how to track the number of users by role over time?
You should record custom variables in Google Analytics. Download the GA Module https://www.drupal.org/project/google_analytics and Tokens
https://www.drupal.org/project/token
in the bottom of the configuration [yoursite].com/admin/config/system/googleanalytics 
expand the SHOWCUSTOM DIMENSIONS section
populate the values with tokens like [current-user:roles]
You can also record an event upon account creation which identifies the type of account
https://developers.google.com/analytics/devguides/collection/analyticsjs/events
you can do a JS event on submit that will record or it probably makes more sense to create a library to record all instances of certain types the way Google Developers is implying.