Are you wanting to have an app that to the other users will still appears in their normal chat app?  If so, I don't have anything to offer.

If you're OK with the whole group of 100+ people getting a different app, outside of North America the app WhatsApp is much more common than using iMessage or android build in messaging apps.  WhatsApp supports pretty much anything Android and Apple native apps support, and can have 1,024 users per group.