|Did you perhaps create an Active Directory integrated domain on the server?|
As for sharing directly, create account on the target computer (the one being connected to) from the remote computers (the ones you'll be connecting from) and use the exact same credentials (username/password) used to logon to the remote computers at bootup. Then, add the accounts to the share and grant them appropriate permissions.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.