Have you seen this error message before:
"The parameter 'addresses' cannot be an empty string. Parameter name: addresses"
We get this when a user has been invited to join a second site collection on our extranet, when their email address already exists for another site collection. They get this error message when they click on the link provided by the system to register. I can send screen shots if that would help, but I wanted to check first to see if maybe you've seen this before and you have a quick answer. Thanks,
David from College Station