Greyed out/inaccessible files on File Sharing server on Mac OS
Everyone, which is about 20 users all with iMac or Mac pros, connects
  to the file sharing Mac Pro using the same registered username and
  password...All the permissions are set to write and read for
  everyone.


Emphasis Mine

This one paragraph sums up the issue you are having; you do not have this setup correctly at all.  What this sounds like is that even though everyone has read/right priviliges, the file gets locked so that only one SMB client can access it at a time.  When two clients access it - even with the same credentials, which client has the correct changes?  SMB solves this for you - the one that got there first.

What you should have is a unique user account for each person that needs to access the file share/server.  If you want to maintain a single login for your users across all servers, look at implementing a directory server of some sort.  I prefer Active Directory as I work in mixed environments; second in line is LDAP.