Facebook group photo tagging?

I've created a closed group on facebook and recently began tagging some members within photos. I was surprised to see that by doing this it gave other facebook users entry into my group (or at least the photo section) even though they are not members. How can i stop this from happening? Or is it up to every one of my groups members to change their privacy settings so that their friends can not see the photos in which they are tagged?
 
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