When a team member is added to your plan, you will be charged a prorated amount for that member depending on how far you are through your billing period. This also applies if a contributor on your plan is changed to a Team Member.
When a team member is removed from your plan, a prorated credit of the per-team member fee will be added to your account, which will decrease the amount of your next invoice.
If you are on a monthly plan:
The new team member will be added to the cost of your monthly invoice. For example, if you are paying $48/pm for 3 team members, adding any additional members will cost $16/pm each depending on how far into your monthly billing period you are.
If you are on a yearly plan:
The new team member will be added to the cost you pay each year, with the amount for the year taken immediately. For example, if you are paying $504 a year for 3 team members, adding any additional members will cost $168 a year each, again depending on how far into the billing year you are.
You can manage and add team members here.