You dont have to leave the group to stop getting the emails. Go to the group home and there are settings at the top of the page, you can either change the settings to stop the e mails or there is also an option to leave the group
Click the 'Groups' tab.
Click the 'My Groups' link on the left side of the page.
Go to the page of the group you want to leave.
On that page, under the description of the group, click on 'Group Settings/Leave this group'.
Click on the blue button that says 'Leave Group'.