Remind.com

Steps to Set Up

1. Create an account
2. Create a "class" - any type of group, large or small, with 1 recipient or 1000
3. Add people to your group
+ This is the true beauty of Remind
+ You don't have to download the app at all to get messages the teams (classes) you subscribe to. 
+ After you create a team (do this on the desktop), Remind gives you the option to help people subscribe to your class in a number of ways: 1) Show people how to join manually, 2) sharing a link (remind.com/join/classname) 3) Add people directly by their phone number. None of that requires them to set up a Remind account nor download anything!

A few tips:
1. Make sure your people enable push notifications
2. Don't send out too many messages! Remind is meant for announcements, not necessarily conversations

The Remind web interface