How to stop friends seeing what you have put in groups on Facebook?

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I post things on groups on Facebook and realised two are not closed groups, i noticed this when a friend had left a comment on something i had post in a group. How do i stop what i have put in open groups coming up on their Facebook accounts?
Yes i could just use only closed groups BUT, BUT one is a special needs groups and is the only one like it but it is a open group.

[abraXus]

if it's an open group, you cannot just turn it off

the only option for you would be for you to add the people you dont want seeing those things to your restricted list, but they wont see any of your activity then

[Daniel]

You Can Not Do That Because There is No Group Privacy Settings For What you Post Anybody that Views the Group Can See What you Have Posted