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Once you expand it you can see the contacts for that client like this, click the blue arrow again to update permissions for the contact you wish to update:
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A pop window will open up and show like this:
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Make sure you assign the correct permissions, check the folder access permitted if you want this user to see the files, then if you want this user to be able to read/write to the files check this setting as well. Once done click Update to save these permissions.