You can mute group notifications for a specified period of time. You'll still receive messages sent to the group, but your phone won't vibrate or make noise when they're received and you won't receive any notifications.
Mute group notifications
Open the WhatsApp group chat, then click the group subject.
Check the Mute box.
Select the length of time you'd like to mute notifications for.
Click MUTE.
Alternatively, hover over the group in your Chats list, then click Menu ( or ) > Mute > select the length of time you'd like to mute notifications for > MUTE.
Unmute group notifications
Open the WhatsApp group chat, then click the group subject.