Go to the SharePoint Site in which you want to give permission to the user. Below are the Example to follow.
Click on Documents as highlighted
Once you select the folder that you want to share.
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.
Enter the email address of the users.
Click on the Icon / Drop down as highlighted in the below image
Select the permission that you to select and click on send to share the folder.