LinkedIn Groups

5. Opt-Out of Notifications When Joining Groups 
 
Joining LinkedIn Groups can be a useful way to connect with people and discover job leads. But if you're actively searching for a new position, be careful: Group owners can enable a feature that sends a notification to others when new members join. If you want to be more private about which groups you join, you can disable this notification.
Navigate to your Privacy and Settings page and click the "Groups, Companies & Applications" tab. Below the "Groups" subhead, click "Turn on/off notifications when joining groups." By default, this setting is automatically set to publish notifications. To turn it off, uncheck the box and click Save Changes

(Information Week)