Adding a staff member is just like adding a client! You'll need their email address and name, and they'll accept it by clicking a link and filling out the required information.
Go to People>Add Profile. When you enter email and name, be sure to select the appropriate Staff role in the "role" field. (Office Manager, Provider, Owner)
Office manager: permission to manage all staff, all clients, all appointments in the practice
Provider: permission to manage all of their OWN clients, their OWN appointments, and start/host sessions, set their own waiting room, etc. (this account type will increase your monthly fee)