Restriction for opened by on HR case

[kumaar]

I want to restrict Hr cases to opened by 

The condition are 

When on the case assignment group is Group1

And the opened by user is not from this group then case should not be visible to the user..

What should be the approach..pls provide the code as well

I tried but its not executing 

[kumaar]

But in this condition if the user is subject person or opened for then case will be hidden..i don't want that..

I only want if the user is in opened by and is not part of assignment group then the user will not see the case