How Groups Work
A Facebook Group is essentially a subset of your Friends list. It isn’t something that your Friends have to join; there’s really no two-way participation. It’s merely a way to send posts, photos, chats, and other files to selected Friends only, instead of your entire Friends list.
When you create a Group, it’s for your personal use only. To communicate with the Friends in a Group, you post an update to the Group itself, instead of to your entire Friends list. Only those Friends you’ve added to the Group see the update you’ve posted.
It’s the same thing with photos and other files. Instead of posting the photos as you would normally, for everyone to see, you post the photos to the Group. Only members of the Group, then, can see the photos – not everyone on your Friends list.
In this fashion, then, Facebook Groups let you direct communications to small groups of Friends, rather than to the dozens or hundreds of people on your overall Friends list.
Note that you don’t have the ability to opt out of another user’s Group. Again, consider these Groups as being similar to mailing lists with in an email program; you can’t control how someone else groups together their Friends. You can only be added to a Group by a Facebook Friend; you’ll then receive any status updates or files that Friend posts to his Group.
You will receive a notification when you’re added to a Group, and that Group will appear in the Groups section in your Home page’s sidebar. You’ll also receive a notification whenever someone posts or shares a file with the Group. Click the link in the notification to view the Group page and the new communication.