In the Android beta version 2.18.335, WhatsApp is adding a new feature that allows you to privately reply a message received in a group. The update, called Reply privately, has been revealed by WABetaInfo, a Whatsapp BETA testers forum reknown for giving spoilers on potential updates to the app.
Currently, you can only have a standard Reply option which is visible to everyone in a group, but the new update will allow you to restrict the message visibility to a certain contact.
We are yet to try out the feature, but the forum suggests that when you tap the Reply privately option, WhatsApp will automatically open the private chat with the contact.
To effect a private reply, you hold the message of the sender to which you have to reply privately, and then tap on the three-dotted menu on the top right corner of the app. Here, you will get the Private Reply option. Select this and the selected message will open privately in the chat window of the sender in the form of a reply thread.

WABetaInfo also states that you should be able to use this feature to privately reply old messages in a group where the Send messages feature is on “Only administrators”, and even if the group is closed.
However, it is as well reported that the beta version 2.18.335 comes with some bugs and eventually “crashes when you try to delete a media in the chat”, and it is recommended that you try out the later beta versions of the app to experience the feature.
WhatsApp is expected to roll out the feature to iOS and the public soon; with direct access available if you sign up for WhatsApp beta or download the APK file.
Recently, a new Stickers feature was added for both Android and iOS app users, coming with 2.18.329 version and 2.18.100 version respectively. The stickers are an extension to emojis and they come in 12 sticker packs pre-loaded when you update your app.