Can not add user to Zoho Invoice

Can not add user to Zoho Invoice

Hi Support,
 
The following mesaage comes up when trying to add a user to zoho invoice.
 
Invitation has not been sent as the user michael.walshe@ is a member of another organization.
 
This person is a member of staff and we would like to be able to have two people send invoices out. Currently it defaults to the one email address.
 
How do we fix this?
 
Thanks Phil