Every folder and file in your Google Drive space can be shared with other users through a link. Whenever someone tries to access this link on an account that doesn’t have permission, they will have an option to request access from you. This option sends you an email like the one shown in the photo.
You can give them access to the material here or simply ignore the email to deny them access. If you believe that you have already shared the material properly, then you should have your users make sure they are using the proper accounts.