Over the past few months, Facebook groups created prior to October 2010 have been upgraded to the new groups format.
Unfortunately, once upgraded, all members of the group automatically receive email notifications by default, even if they didn’t subscribe. To unsubscribe from the group’s emails, there’s no need to leave the group, just go to the group’s page, click the “Edit Settings” button (at the top right), and unselect the two tick-boxes “Email notifications” and “Send me group chat messages.”
Alternatively, you can opt out of all notifications by opening:
Account >> Account Settings >> Notifications
If you administer one or more FB groups, you may find the following links useful: