That's probably the message with key"invitation.invitesender.email.subject" in invitation-service.properties Which has a value of Alfresco {0}: You have been invited to join the {1} site.
Ok cool. So i suppose if these items are spread out among different files to edit the subject of other of other emails then I could just grep the folders for whatever the the subject says and edit the results.
Where would the customized properties files be stored. I have tried \Alfresco\tomcat\shared\classes\alfresco\messages and \Alfresco\tomcat\shared\classes\alfresco\extension\messages but after a restart the subject is not changed. I have tried both the invitation-service.properties and the invitation-service_en_US.properties.