HOW TO DISABLE EMAIL NOTIFICATIONS

Option 1

Click "Notifications" - top right hand corner
You can either click "Off" and get no notifications at all
Or click "Friend's posts" to only be notified when they post

Or you can follow Option 2

Option 2

Click "Notifications" - top right hand corner
Click "Email settings"
You will be taken to FB Account/Notifications/Groups
UNCHECK all the boxes under groups or you will be inundated with emails

The option is yours as to which notifications you would like to receive - I would personally tick the "Friend's posts" as it will keep me in touch with what my friends are doing.