All it takes is "And", yes say that word when you plan to deliver or send message to more than one persons. Like to your mother, father, wife, children or friend.
Here is how you do it, let's say you want your message sent to 4 people. Then dictate this line to Siri:
Send message to my mother and wife and son and James.If Siri responds your dictation with this:
What do you want to say to these six people?It means you are on the right track, you can continue dictate the rest of the message and Siri will be sending it to the said names at the same time. Moreover this method can also be used to send email to multiple persons at once. The keyword is "AND" don't forget that, because it will save your time.
Source: When Will Apple.