Sharing a folder only to a user on another machine

I need to share a folder on a machine to make a scheduled backup from a server. However I don't want everyone to have access to this folder.
How can I give access to the server Administrator only?

i.e.: ComputerA and ComputerB. I want to share a folder on ComputerA and user \ComputerB\Administrator should have access to that folder on ComputerA.

Thanks!