You can clients either from templates on the right side where there is a list of clients, or by selecting the Clients menu and doing the same there.
Click the sign at the top and complete the fields to add a client.
You can add as many clients as you want, and these are all private to you. No email is sent to those clients until you assign homework within a program and click send homework. Once you do that they will receive the login information and be able to access the system to update their homework and track their progress.