Staff are people in your overall practice account umbrella. You are either paying for them as part of your plan (i.e. another provider in your practice) or they are an office manager for you.
An office manager can schedule and add clients on your behalf but cannot start an online session.
Another provider that is on your account will have their own schedule and list of clients. However, you are able to "share" clients within your practice.
To assign a client to a provider that is a staff member on your account, go to People, invite the client (Add Profile) using their email address/name and then select a provider that is on your account in the drop down menu. They will be assigned to them and visible to the provider's account.
Now the provider will see this client in their list of clients and they will be able to schedule an appointment with them.
If the client is already in the practice, locate their profile under People>Clients. Then, you can simply click on them and find the button for Add Provider. Add an additional provider in your practice and now they are available to be scheduled with that provider.