Group Messages requirements

To send a group message via iMessage, everyone in the group must have a phone number or an email address registered with iMessage.

To send a group message to people who have a phone number but aren’t registered with iMessage, you must have an iPhone with iOS 8.1 or later and set it up to get SMS texts on Mac. See the Apple Support article How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac