Two quick commands to see who is currently logged in, and who logged in previously:
>show global-protect-gateway current-user
>show global-protect-gateway previous-user
You can also specify the username with each command to see specific results.
If want want to force a user logout (from my testing, the user will not see a notification they have been logged out):
>request global-protect-gateway client-logout domain yourdomain reason force-logout computer computername gateway gatewayname user username