When a user is added to your plan, you will be charged a prorated amount for that user depending on how far you are through your billing period. This also applies if a guest on your plan is changed to a Team Member.
When a user is removed from your plan, a prorated amount of the per-user fee will be added to your account as a credit, which will decrease the amount of your next invoice.
Team members = paid
Example - If you are paying $16/pm, per user then adding an additional user with cost an extra $16/pm per user. This amount is pro-rated.
Guest members = free