I don't know if there is an app already for this, but if you're just going to go off logs that tell you when people log on and log off, then you will need to maintain a state machine using lookups. I was pointed to this a while back and found it useful:
http://blogs.splunk.com/2011/01/11/maintaining-state-of-the-union/
This would require a good understanding of the right logs and events to parse through to find the information you want..
Alternatively, you could set up a script to run when a user logs on/logs off that writes their username/IP address to a file.. which you can then Splunk. That's a bit of a hack though.
... View more