When you add someone to your plan you will be charged a prorated amount based on your current billing period.
If you are on a monthly plan:
The new user will be added to the cost of your monthly invoice. For example if you are paying $70/pm for 5 users, adding any additional users will cost $14/pm each depending on how far into your monthly billing period you are.
If you are on a yearly plan:
The new user will be added to the cost you pay each year, with the amount for the year taken immediately. For example if you are paying $660 a year for 5 users, adding any additional users will cost $132 a year each, again depending on how far into the billing year you are.
When you remove a user it creates an empty seat on your plan which you fill at a later date. Any unused time will be credited to your account and applied to the next user you add.