This is very irritating. Last year I was invited by a third party organisation to their box.com instance so they could share some documents. My own organisation has just procured box.com account. I need to work on an urgent project using box but my own organisation can't join me as a user because I am already linked to a third party. This is an insane scenario. They have tried multiple times to send me invitations to collaborate but they are not coming through. Utter madness. The only solution I can see is to setup an alias email account, which seems a frankly absurd suggestion.