That relates to the users on your file server.
When you setup a share in windows (and probably any other OS) you are asked if the share is open to all or what users has what rights (Windows usually allows you to skip this more or less).
Anyway, the user you must add is a user with full read and write privileges on the share.
If "guest" or "everyone" has full access you don't need to put in anything.
I usually just put in my Windows user anyway.
Not sure if we talk about same things. I'm talking about samba share from LMCE Core machine. Public share, with public media on it, should be I guess accessible from all windows machines without any password - or am I wrong ?