The ability to customize and save invitation text is a standard feature. It keeps you from having to re-craft or re-create a custom invitation.
To do this, go to the MANAGER tab and select the "Invite Members" sub-tab. Scroll down to the "Write your invitations" area and:
1 - Click within the text box to edit the existing text.
2 - Click the "Send Invitations" button.
3 - A pop-up window will appear. Type the name of your new invitation in the MESSAGE NAME box. Click SAVE AND CONTINUE.
4 - Simply select this custom invitation from the drop-down list under "Choose Message" the next time you want to use it.
Note: you can create and save custom invitation text. What you can't do today is make your invitations default to this customized text.