How do you set up such a group message?

[AC Question]

I was at a rally last year and they offered the attendees to join a group text to get notifications of each event taking place during the rally. There were likely 1000+ people receiving these messages. We had an option to opt out at any time. How is this done?

 

[B. Diddy]

Welcome to Android Central! Was it a group MMS, or were they using iMessage (for iPhones)?