Facebook privacy about groups?

[Maria]

I have "registered" in an open group in Facebook and my sister (who is not a member) told me that when I write something on the group's wall, my post appears on her News Feed. I don't want everyone to know that I'm a member there! I didn't know that this happenned because it doesn't appear on my profile or my initial page and thus, I can't select "Hide post" or something. What can I do to not let this happen again? Is the only way to not write again on the group's wall?

[abraXus]

yeah it appears in your friends' tickers, and you cant shut it off, because it's an open group - there is nothing you can do to hide that activity unless you add all your friends to your restricted list