You can send email invitations to known potential affiliates from the affiliates page. Instead of re-writing your message every time, you can set a default message in your settings. Access the settings page by clicking on the cog icon in the navigation menu on the left or using the drop down menu by clicking on your name in the top right corner. (You may also click here.)
From the settings page, scroll down to the "Default Invite Message" block. Fill out your default invite message and click the "Save" button to commit your changes.