I'm not sure I'm right about this, but I don't think there is a way to audit logon events, at least there is nothing in the P8 KC. And I would say this is because of the way the CE works, the JAAS context is actually built against the server, then calls are just authenticated and authorized, there isn't really a logon phase.
However, what I'm sure of, is that it is definitely doable on the application side because there, there is a logon action initiated by the user. How do your user connect to your platform? Are they using Content Navigator, Workplace XT, a custom application you developed or the Java API, or maybe all of them? Please provide more context.
For instance, if they are using Content Navigator, then a simple plugin listening for the login event and logging this event would to the trick.