To add an user use add user (see man adduser for details)
Do you want the user to be able to log in ? You may don't want to create a home directory for it, so you may be interested in the --no-create-home option.
Then use chmod and chown to set the permissions. Something like (see man chown and man chmod for details) :
chown username filename
chmod u+rw filename
Note that you won't have only one
file whith read write access for the user on the whole server, indeed there are world readable/writable files (wanting to give access to only one file on the whole server for an user seems odd to me)
Hope it helps.