Group Messages requirements

To send a group message using 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, MMS and RCS texts on Mac. See the Apple Support article Forward SMS/MMS/RCS text messages from your iPhone to other devices.