Within our SharePoint farm we have site collections with many sub-sites and are in the process of introductin ECTS into this environment. We have now come across a problem relating to the sub-sites. If we try to add an external user to a group created at the sub-site level and error is returned - "Value cannot be null. Parameter name:url". In the sharepoint logs we seem to have an associated error "The user does not exist or is not unique". Adding an external user to a group defined at the site collection level has no such problems.

Is this a limitation with the toolkit, or is there something we have configured incorrectly.