Yes, Administrators can add a custom message to the invitation email that is sent to new users. Navigate to the Group (where you intend to add users) and click the Actions menu, and then Edit Group.
This will present a screen where you can edit the name of the Group, but more importantly, add a custom message that is added to the user's invitation email. Once this is saved, your message will appear in the invitation email for new accounts as a paragraph above the heading "Your action is required" (see here for example email: http://i.imgur.com/IVcZ1KE.png).
Note: Each Group can have a unique message if you desire. All users invited to this Group will see the same invitation email message.