If you're the project manager, you can customize the default invitation email from the Project settings. (You will always have the option to further customize an email for any specific invitation.)
From the Account Management menu, click Projects.
Click the name of the project to open its settings.
Click Edit to enable editing project settings.
Under the Self Service settings, find and check End Users Customized Invitations.
The Custom Invitation editor appears a little lower down the page.
Make the changes you want and then scroll down and click Save Changes.