I use an app called Connect. It looks great and seems to work well. I've only used it a couple times now for sending out text messages but seems to work well.
Connect - Contact Manager for iPhone by Dexem https://appsto.re/us/4WItN.i
The iPhone messaging app will let you send messages as described in the support link, however, if you send them regularly to multiple groups you will want to use the app that I linked above.