To see when someone logs on you type on the command bar under your Main Console: +notify Eendje (if Eendje is username of that person). This adds Eendje to your notify list. When she logs on and off you will be told.
To remove someone from your notify list type: -notify Eendje
To see all usernames on your notify list type: =notify
To see those currently online type: znotl (Or open your notify window. Menu above: Window > Notify List.)
For more options, like being notified for a titled player arriving or for games of a specific player starting, read "help notify"