To reply to a specific message in a group chat, simply swipe left on it, type your text and hit ‘Send’. If you tap on the quote in a message that is a reply, the app scrolls up to the original message – and shows an arrow button to go back to the previous location. This makes navigating conversations in groups easy even if you've been away for a while.
The person you replied to will be notified about your message even if they muted the group chat — their notification settings for you personally apply in this case.