If you're talking about users that you see on AMP Setup -> Users...
Then you can trace back a user's last authentication to the AMP GUI in at least the last 4 days. To do this for the current day, you need to login to AMP from the CLI and parse through /var/log/httpd/access_log.
If you cd /var/log/httpd, you will see that AMP retains 4 days of access_log (that's including the current running date). The access_log is an alias to the current day's log file. You can try grepping the logs for specific usernames.
# cd /var/log/httpd
# grep -l admin access*
-This will output any access files that the user admin appears in
-You can then use a text editor to view the file and trace down the time of the login.
Format of the log messages is:
AMP_IP - amp_auth_username [date/time of request] "page requested"
Example of access_log where 'admin' is the logged in user:
10.10.10.10 - admin [17/Sep/2012:05:07:42 -0700] "GET /amp_stats?include_all_stats=1 HTTP/1.1" 200 922
10.10.10.10 - admin [17/Sep/2012:05:07:42 -0700] "GET /folder_list.xml HTTP/1.1" 200 3825
10.10.10.10 - admin [17/Sep/2012:05:07:42 -0700] "GET /alerts.xml HTTP/1.1" 200 233