Whenever I make any comments in a Facebook group or do any activity in a particular group, all my friends get notifications for that update, even if they are not part of that group. How can I stop my updates from them?
The group needs to be private (closed or secret), otherwise they publish updates.
Something you can do, which I know isn't ideal, is to delete from your time-line the publication of the group update. Of course, if one of your friends just refreshed his stream before you got a chance to delete it, they might see it but it's better than nothing.