REGISTER or login:
Logged onto what?
A server, and application?
I have about 20 people still using our AS/400. My Boss wanted to know how often, (in the last six months,) they logged on to the AS/400 to pull information. We have an old 9406 AS/400. I haven't had to track user log ons before so I don't have anything set up to do it.
If no actual tracking exists, you can only use guesswork from unverifiable sources. For example, you might list all messages from the system history log that show jobs starting and ending. The messages tell you nothing about whether a "logon" happened nor what the purpose of the job was, but it's unlikely that some of the jobs have any meaning other than that.
For example, you can run a DSPLOG command to list all CPF1124 messages to see every job that started during a timeframe. I can't guess how long your system is configured to keep history log messages, but it probably goes back at least for 30 days.
There will possibly be many more CPF1124 messages than you care about, and there is no good way to filter them down without some programming. You probably would only be interested in jobs that have an unqualified job name that matches an interactive device description name. Then you'd only want ones that show a job-user name that matches one of your 20 users. The combination of interactive device name and a general user name probably indicates a green-screen "logon" happened. (But job names can be assigned, so they can't be guaranteed.)
Logons can also happen in many other ways. Maybe you don't need to care if information is pulled through ODBC for example or through other servers. If you're only going to check green-screen interactive logons, then DSPLOG for CPF1124 and counting the ones that match the "device name" pattern is probably the best you can do.