step 1:- Download and Install FB App from HERE.
step 2:- Open it and Log in with your Account.
step 3:- Now click on group icon present in 3rd row, now open the group in which you want to add your friends.
step 4:- Click on GROUP INFO > Add members.
step 5:- Mark your all Friends and click on done
now you have added your all Friends in the group .
