Unfortunately, you can't set logon hours through group policy. You'll have to set them specifically for the individual users. Luckily, as Tribus mentioned, you can select multiple users and set those hours for each user. The second part is enforcing those logon hours, which *is* done through group policy. Specifically, in Computer Configuration\Policies\Windows Settings\Security Settings\Local Policies\Security Options
You'll want to set "Network Security: Force Logoff when logon hours expire" to Enabled to kick your users out when they get past the point that they're supposed to be there.
