reporting on sessions by contact

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We have one interface where we have the users log in when they access the site. On our contact records for these users that log in, we have custom fields to show the location and department of every contact.  What I'm trying to report on is the number of sessions created in the site each day by department and location.  But I can't find any way to join the stats table with the contacts table in order to do this.  Is this even possible?  Has anyone else even tried to do something like this?  The User ID that is in