Within SecureDrive permissioning is done at the Drive level. Any user that has access to a drive will have access to all files within the Drive. To ensure you have the correct users accessing the correct drives we recommend segmenting the drives by User groups. So lets say your HR team has a need for a drive, you would create an HR user permissions group and add all your HR members to that permission group, from there you'd create your HR drive in SecureDrive and, using the User group drop down, you would select the user group for the drive and add all needed members there.
For more information around user permission groups see this article.