Because the only login fields tied to the API are Last Login Date, Login Count and is Login Blocked, I'm not sure the system can go back and track user logins over the past year.
Audit logs track the past 90 days of user logins, so if you were to set a reminder every quarter to check, then you'd be able to count logins for all users and export that data into Excel to remove duplicates.
Another idea would be to run a user report on December 31 (granted, you'd have to set a reminder) that filters for Last Login Date > This Year, that would provide you with a unique list of users who logged into Workfront. More information on how Workfront views login information here.
