You will be charged for all team members on your Team page except archived. Account owners are free unless they track their own time. For example you have 6 people on your Team: 5 employees (1 of them archived) and yourself - you will be charged for 4 people assuming you do not track your own time.
The fees are prorated – if you add an employee for a week, then remove or archive them - you'll pay for that week only, not month. You are free to add or delete employees, archive or restore them any time as you see fit.