Select the Access Tab - this is where you give access to the library to the readers. This access is done at the folder level.
To give other readers access you can either click on the Add button or click on Access same as parent folder. This will bring the same access rights from the folder above to this folder.
If you click on the Add button you will see a new window contains current users list who do not have access to the selected folder. You can either select from the current user list or click Add New. Add New means that you can invite external people to access this folder.
You can determine the level of access: Readers can only view documents on folders they have access to. Librarians and Assistant Librarians can be given View or Edit access on a folder.