As you might know facebook has recently rolled out the new facebook groups which are open and anyone can add you in those groups without your permission, this is nothing but just spamming I wonder why facebook and allow all this to grow their network and reach to their advertisers.
1. Login to facebook.com
2. Once you are logged in, go to your facebook profile home page
3. Now click the link See all in the left pane, under the facebook groups as shown in the image below.
4. Now Scroll down to the bottom of this page, and locate My Groups and click the cross icon to remove or delete a facebook group, this will automatically unjoin or leave the group as shown in the image below.
5. Once you click to remove, it will ask you to confirm it as shown in the image below, so click the Leave Group button and you are done.