Go to the group text message. Tap the top of the conversation, then tap the Info button . Turn on Hide Alerts.

You can also swipe left over the group text and tap the Alerts button , if the group text isn't pinned. When Hide Alerts is on, you'll find a crescent moon icon next to the conversation.

Hide Alerts stops notifications for a specific group text, not your device. You'll still receive all other messages and notifications on your Lock screen. Learn more about managing your notifications.