This article is in the Control section of the App.
In order to customise the proposal invitation subject and message, go to Control and then click on Company>Settings.
From here click on Email settings and you will be presented with the options to customise the subject and message using a text-editor.
You also have the ability to set the SMTP settings which allow you to choose where it looks like the email invitation has been sent from. Find out more about how to set this up here.
Ensure that once you have made your changes, click the Save button at the bottom of the page.