Yes! You can add a welcome message to your group that is sent to all new members automatically. Click the "Welcome Message" link in your "More Admin Tools". Then, enter the message you'd like to send and check the "Send this message to new members" box. Click the "Save" button and your message will be sent to all new members of the group.
If you decide later that you no longer want the message sent, uncheck the box. You can also edit the message at any time.