You can identify the logon and logoff syslog entries (e.g. 611101 and 611103 (click for details) are a good start) and create a logging message list that only includes them. You then send the syslog messages to an external server using the logging trap <message list> option. The Configuration Guide covers these options here.
The external server could be a commercial syslog product (e.g. Kiwi syslog server) or a simple Linux host running rsyslogd. The commercial products will have the nice things like being able to create and email reports. The Linux option will have a simple text file that you can review.