In version 4.0, there is an audit report that reports logins. See Scheduled Reports.
To see a history of who has logged into the controller for pre-4.0 versions, you can query the audit logs from the command line or use the Python SDK for the Controller REST API.
1. Bring up a command prompt and navigate to and execute this command: <ControllerInstallDirectory\bin\controller.bat login-db
2. You will see the mysql> command prompt in the command line window.
Execute this query, which returns results for the last 24 hours based on ts_ms field:
select user_name,FROM_UNIXTIME(ts_ms/1000, '%Y %D %M %h:%i:%s') as login_time from controller_audit where action = 'LOGIN' and ts_ms > (select max(ts_ms) - 1000*60*60*24 from controller_audit);
Check out the Python Extension on AppSphere. The examples folder contains a python script that generates a report. The file is audit.py.