Group chats in WhatsApp are great for communicating with multiple people at once, but that can be a blessing and a curse — any message you send will be seen by everyone, and that's not always ideal. Thankfully, there's a new feature that lets you respond to group messages privately.
As of version 2.18.380, WhatsApp gives you the ability to privately reply to a group chat participant straight from the chat itself, doing away with the need to start a separate conversation from scratch. The feature is available for both iOS and Android devices, with minor variances in how it's implemented, but I'll outline any differences below.
Now, when you want to reply to a message in a group chat, but you only want the sender to see your response, start by long-pressing their message. From there, either tap on "Reply privately" from the prompt (iOS) or tap the three-dot menu button, then select "Reply privately" (Android). Input your message as you normally would, then hit the send button.
Regardless of whether you're on Android or iOS, any private discussion you have with a group chat participant will be located in a separate conversation page away from the group chat itself. The person you replied to can keep the conversation private by posting his or her replies within that chat page, which ensures private messages aren't accidentally posted in the group chat for everyone to see.