Domlog for authentication on web.   Server log for client.  You can look at users mail file for activity

What you will want to look at is User Licence Tracking

https://www.ibm.com/support/knowledgecenter/SSKTMJ_8.0.1/com.ibm.help.domino.admin.doc/DOC/H_LICENSE_TRACKING_4561_OVER.html

It will log each time a user authenticates with a Domino server on either HTTP, IMAP, POP3, SMTP,  LDAP, or DIIOP.  You will need to enable it on all your Domino servers (via the configuration document) and the information is placed in a database on your admin server (userlicenses.nsf).

You may need to do a bit more work to use the data in a meaningful way but there is a field called TImeStamp in each document which is the most recent time the user authenticated with the server.

The only downside I have found is that Traveler users don't appear if they are using Google Cloud Messaging to sync so I add Traveler sync data to get a fuller picture of usage.

I've had License Tracking set up for over 10 years - My experience is that it only updates any given individual once a month.

So while it's useful to get a total count of users, and who is FAIRLY current, (within 30 days) it's not so helpful to find out who hasn't connected for say, a week.

Brett, while this isn't as timely as I'd like, it has been the most helpful tool that I have found.