To create an employee without an e-mail address, go to "Employees" in the menu. Then you will see a blue button on the right with "invite employee". If you click on it, you can also create employees without an e-mail address.
Another possibility would be to create subaddresses of your own e-mail address. This works as follows: email@example.com, firstname.lastname@example.org, email@example.com, etc. Instead of a number you can also use the name of the person.