To Join/Quit a Specific Group
1. Go to that group's home page. If you don't know where that is, you can find this by searching for the group on the main Groups page. (Search bar is at the top-right of the list of groups.)
2. Once at the home page, the "Quit Group" or "Join Group" buttons will display based on whether or not you're already joined.
Note that the "Quit Group" button is not very prominent. This is to avoid people clicking it by accident.
3) Click the button. You're done.
1) you can click the "My Groups" link -- below your name/username and photo (towards top right):
To quit multiple groups quickly
2) The page that comes up displays the first twenty groups you're currently joined to. Click the "Quit Group" buttons to the right of their listings. Be aware that there can be multiple pages of listings. Be sure to click forward to further pages of groups if necessary.
You can also easily set your emails settings using the email settings links (right above the Quit Group buttons).
Joining More Groups Quickly
If you're looking to join more Groups, go to the main Groups page and use the same style of buttons to join the groups you want. Note that you can search for groups using the search bar near the top-right of the list.