If you want to see who logs in, but might not log anything, then this could be a bit tricky as the records LDSD uses to record user sessions are hard deleted. These do contain when the person logged in and so on, but disappear. If you have access to the event manager module, then you could look at configuring it to produce an event manager entry on create and delete on the user sessions. This will cause an event manager table entry to be created each time someone does this and you could then report on this.
While this isn't too hard to do, it's not that commonly done, so I'd also ask your local support team to see if they have ideas too.