A quick way to monitor users login/logout on your system is to log the STATUS value (i.e. ACTIVE or blank) from the }ClientProperties cube.

View->Display Control Objects
Cubes -rightclick- Security Assignments
browse down to the }ClientProperties cube and make sure the Logging box is checked
tm1server -rightclick- View Transaction Log
Select Cubes: }ClientProperties

All the transactions are stored in the tm1s.log file, however if you are on a TM1 version prior to version 9.1 and hosted on a Windows server, the file will be locked.
A "Save Data" will close the log file and add a timestamp to its name, so you can start playing with it.

/!\ This trick does not work in TM1 9.1SP3 as it does not update the STATUS value.