If you need to give somebody access to a particular document centre or library
First find the document centre in sharepoint admin and open it. You may need to give yourself admin access to view it.
Go to site settings>site permissions to view who has access. Here permissions will be split into Members, Owners and visitors. You will be viewing permissions for the whole document centre.
Specific Libraries within the documents centre can also have their own permissions. To view these click on a library>settings cog>Library settings>Permissions for this document library
Certain folders within each document library can then have further special permissions on too. The "Show these items" link will give a list of which folders have special permissions and who has access to them.
