I have not seen any notifications or posts in my newsfeed from any friends when they post to groups that I have not joined. I am not saying it doesn't happen, I am just saying I have not seen it.
But what I have seen is recommendations to join groups that friends have joined, and this can be a little weird at times, seeing some of the strange groups that some of my friends and family have joined.
And yes, Facebook does tell you who has joined what group. So, you might want to think twice about joining any sexual fetish groups on Facebook, if you don't want all of your friends and family knowing about it.
Also, watch out for pranks, where friends add you to a group you would rather not be in and have advertised to your friends & family. (Yup, even if you don't voluntarily join those sexual fetish groups, a friend of yours can add you to one of them as a joke, and all your friends & family will see your face and name associated with that group, recommending that they join too, on their right sidebar)
http://www.themarysu...s-forced-add-nambla/ 
