Custom notification for whatsapp group
The best way to do this is to do the following:


Open WhatsApp
Long press on the chat that you need to set the custom notification for
Press Group info
Press Custom Notifications
Tick the checkbox at the top right
Change the notification tone, vibration pattern, LED light colour as required.


For what it's worth, I am using WhatsApp version 2.12.367, on an Android 6.0 device, but I doubt that it is dependent on Android version.