When trying to add a user to Marketo we get 'User creation errors - "This userid is in use. Choose another email" ' due to the fact that email addresses need to be unique across ALL Marketo instances.
One creative way to get around this is to at a '+tag" to the email address befor the '@' sign. For example if we get the error adding a user record for 'Name@domain.com' we can simply use the '+' sigh and a tag of your choosing (in this case '1') and use the email address of 'Nameemail@example.com'. The system is happy as this is a unique email address yet all emails will still be delivered to the appropriate inbox for 'Name@Domain.com'