The only thing you can do is to go into the Whatsapp group you want to mute. Then press Menu > Mute > 1 Week. That's the longest you can mute a group.

Unfortunately, it will still display a notification in the notification bar. But it won't vibrate or make a sound anymore.

The only thing you can do is to go into the Whatsapp group you want to mute. Then select Menu > Mute > 1 Year and uncheck Show notifications.