While you can export the User list to see the last activity date, it would be better to also display the date in the Users (Billing) page in Aha!. If there's a concern about taking up too much width on the page, you could add a toggle for this and by default not show the last activity date.
This would help with periodic review of the accounts to identify accounts that are no longer actively used. Then we can inactivate accounts, due to inactivity in the past X months, and reallocate a paid seat to someone else who needs it more.