Error message when adding a user: "This Email address is already taken"

Error message when adding a user: "This Email address is already taken"

Hi, I have a zoho people account and I m testing a zoho recruit, but im not able to add user in the trial version. Error message is: "This Email address is already taken. Specify a different Email address for the user." How can I add a user who is already existing on my Zoho People account? ThnaksSerge