So you are trying to add a new user, and Timetastic says they already exist, and yet they don't seem to exist on your account.
What's happening is that your new user has already created an account on Timetastic by completing the Signup Form. And in doing so have created an entirely new company on Timetastic, separate to yours.
To move forward they'll need to cancel the account they created.
Once they are in the app, they can navigate as follows:
Settings > Billing > Cancel Timetastic
And once cancelled you can add them to your account as normal, through the 'Users' section.