When you manually invite participants, they will be sent an email with their access link. You can customize this by adding your own banner and some custom text.
To do this, head to the admin pane, click on the Settings tab, scroll down to the Emails section and click Edit:
Enter your custom text in the Invitation message field. Line breaks and formatting are not supported — your message should be just a single paragraph. You can also upload a custom banner by dragging and dropping or browsing for a file via the Invitation email banner field.
Below is an example of what the invite email looks like with a custom invitation message added (the text outlined in red).