To grant access to a file or folder only for educators, for example, you have to right-click on the respective file or folder and then select „Get info”. This will open a window with the details of the document and different other options. Choose the option "Publish file". Then you will need to select the group you want to publish the file for.
To give preferential access to a file, right-click on the respective file, then select „Get info” and the push „Edit” button. Then another window will open from where you can select the person or group/s you give access to, but also modify the name and the description of the file.