We use K2 to perform Authentication. However, the data for this is locked away in the Licensing User smartobject, and there is no way to get the data out to churn user reports on users and last access date. Should also consider expanding license user to include the workflow they access. If there are APIs that can allow us to get/pull data from this table, it will help ease the work required to separately track the last access date.
With 5.7, we've added the ability to export the licensing reports as Excel files, so that you can look through the data more easily.