how to change settings so that fb won't publish your group activities on news feed?

Hi there,

Anyone knows how to tell fb to stop publishing your activities on groups to news feed?

for instance when I comment on the wall of a group (X):
"yo, your classes are amazing!!"

My friends, who are not even group X members see:

my name -> X
"yo, your classes are amazing!!"

how do I stop that from happening?

Thanks