How do I set up a large group messaging?

I was at a rally last year and they offered me to register for group messaging to get an alert for all of their activities. I believe I gave only a phone number when I registered. We had the option to opt out at any time. Text messages were sent out to likely 1000+ people. I am sure they were being sent to both android and iOS. How do you I set this up?

You would do this in your contacts. Instead of new contact it would be new group. Name your group and add from your contacts whom you would like to be in the group.