Within Teamwork Projects, when you add people to your site as users and send them an invite, it includes a link that they can use to access the site and set their password.

You can add a custom message to this invite by going to your site settings. 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. 

Here, you can click the Set Message link to show a text box where you can add your message. You will see a preview of the content when you click the Save Template button.