Frankly - I would enable accounting for my radius server and pull the data from it. That would be a very easy way to get that information. You can get it from the logs but will have to write some filters either within the SSL box or externally on a syslog server to isolate out the login data.
Radius logs would be more appropriate to look for activity.
Alternatively you can filter user access logs with specific codes - you can then export information into excel and sort.
Its a tedius method as well. Dashboard graphs (System -> status) show you concurrent users .