How do you ask Siri to message whatsapp group?
I figured out today how to do it. 

First, you need to be sure that the name of the group can be easily understand by Siri. As I know from myself some group names are too complex for Siri. It will be simple if the group name is like a person's name. 

Second, you need to add the group name to your contacts without any number etc. just create a new empty person with the group name.