If you add seats in the middle of an annual plan, you will be charged the prorated fee per seat from the time you added them until the end of the current billing period. When you remove a user, the seat will be available to give to another team member at no additional charge. However, there are no prorated refunds or credits for removed users.
As an example, let's say you're on Hubstaff's Team plan, which costs $10 per seat per month. If your billing period started in January and you added a new member in July, you would be charged a prorated amount of $60 for that seat (covering July through December).