invitation: option to not try and add user to external subjects source if they exist

We would like to use Grouper's invitations, but, in our case, our system will force all users to register before getting to any of the apps.  So, when a user arrives at the Grouper UI to accept an invitation, they will always exist in the main/ldap subject source.  We'd like Grouper to check to see if the user exists in other subject sources before adding them to the external subjects source (which triggers an error when they already exist in a different source).