Legacy part of the Advanced Nagios Plugins Collection, download it here.
Just run this plugin on any Unix/Linux machine with the standard Bash shell installed and it will list all logged in users.
You can set the plugin to alert if certain users are logged in, or even better, if any user not from a defined whitelist is logged in.
You can also alert if a specific user or users are not logged on.
./check_users.sh --help for details on switches.
Simple to use.
File issues on the GitHub issue tracker.