A new employee memo not only makes the new person feel welcome, it serves to notify the rest of the organization about a change. Include the new person's job responsibilities and location, as well as a little about where he/she came from.
From: Jim Karter
Subject: <subject of memo>
Please join me in welcoming <name of new person> who <job description>. <First name> <background.>
<Optional additional information.>
Please introduce yourself to <First name> and make <him/her> feel welcome at Abc Company.