Within Teamwork, you can add a custom message to the invites you send to people you are adding to your site.
Click your profile icon near the top right corner of your site and select Settings from the dropdown.
In the settings area, go the Templates tab and choose the Set Message option beside the Welcome Email Template.
A modal will open with a text area to add your message. You will see a preview of the content when you click the Save Template button.
For more information, see: Inviting People to Your Teamwork Site