Assuming you mean that you have the office running on an Exchange server, right-click the calendar on the new employee's outlook or have an administrator open up the new employee's acct and do the same. Go to properties, then permissions (or sharing...I don't have Outlook on an Exchange server here to check...it's the last tab, in any case). Add the HR rep's username and give her him/her the necessary read/write permissions via checkbox.
~Ladi