How to add limited access permission on external users on a document library

Hello,

Thanks for your reply and sorry for the delay on my response. I have created a permission level to grant only basic access on a document library. i named it List_Only so whenever i wanna give access with custom permission level setting on a sub folder, i follow this procedure.
The only problem i have though is that on document library permission settings and/or sub folder permission settings, those users does not appear on the list even though is it validated that their assigned permissions are working correct. I am just wondering what i have to do if i want to remove a user from the library or subfolder.