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Go to the SharePoint Site in which you want to give permission to the user. Below are the Example to follow.
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Click on Documents as highlighted
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Once you select the folder that you want to share.
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Click on the Share Icon that is highlighted in the above image.
When you click on share you will be redirected to a pop-up as in the below image.
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Enter the email address of the users.
Click on the Icon / Drop down as highlighted in the below image
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Select the permission that you to select and click on send to share the folder.