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 £20/pm for 10 users on the Essential Plan, adding any additional users will cost £2/pm each depending on how far into your monthly billing period you are.
If you are on a yearly plan (discounted price):
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 £216 a year for 10 users on the Essential Plan, adding any additional users will cost £21.60 a year each, again depending on how far into the billing year you are.
When you want to remove a user, the users seat will stay active until the end of the current payment period. You can either invite a different user to this existing user seat or delete the user immediately.