How to force group text to use SMS/MMS so all recipients get all messages?

I have a group text thread with 20 people, a mix of phones. When I text sometimes it goes through as an iMessage- other times it goes through as SMS/MMS message. When my (and other iPhone users) texts go through as an iMessage the non iPhone users obviously aren’t seeing the texts. How can I format the thread to force all messages to go through is a text rather than iMessage?

[Re-Titled by Moderator]