When you try to create an account for an employee, you might get an error message stating that there is already a member with that email address even though you can't find it in your employee list. Most of the time, it happens when the contract has ended and the employee's account is in the "inactive" profiles.
To reactivate the profile, go to the "Inactive" section on the profile of your employee and reactivate his access at the bottom of the page.