To leave any group on Facebook just follow these simple steps which is demonstrated in detail with Pics.
Go to Facebook.com and sign in to your account.
Now you have your Home page in front of you open the group which you want to leave.
Click on the settings picture in the end of the options at upper right corner after the "CREATE A GROUP" option and go to the last option i.e. "LEAVE GROUP" and you have done. Know you are no longer the part of that group.