If you go to the group page, under the group icon in the upper left hand corner, you should see a smaller icon of yourself. When you put your mouse over it it should say "Member". If you click on that a list should come up. The last option on that list should be a little blue "crying" icon that says "leave this group" or something similar. (I will note that this function is not working presently, but hopefully DA will have it solved shortly.) I hope that helped.