Does anyone know how can I add posts (mostly photos) to groups which I'm a m member of , but the post wont be shown on my friends news feeds ?
Yes there is a way. You need to set privacy for your friends. Also when you are posting the picture, you can click on the privacy settings of the post, and it would ask you that who you want to show it to. You can even exclude all the people who you don't want to see your pictures