|simplest solution is put the file in its own folder [not nested] and do the share permissions as read write.|
The problem most folks have with file sharing and rights is they don't look at it from the perspective of the client but view it locally.
What I mean my this is you have a volume. In that volume you have folders. In those folders you have files. You see all of this locally. Now if you share a folder, which shares the files, your users looking from the outside can only see the share and the files it contains.
Given your criteria above you would have to make the share with Modify for all.
You would then have to mark EVERY file you don't want having read/write access a read only using ntfs permissions.
See why I recommend you organize via share level then file level?
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