If you have quarterly subscription reconciliation enabled (default option for new and renewing subscriptions after Aug 1, 2021), users added during a quarter will only be charged for the remaining quarters of their subscription term as opposed to the full annual subscription fee(s) with annual true-ups. For example, if you add 50 users to your subscription during the third quarter of your subscription term, the 50 users will only be charged for the fourth quarter of your subscription term as opposed to all four quarters as per annual true-ups.
If you do not have quarterly subscription reconciliation enabled, add-on users will be charged annual true-ups for the full term during which they were added. For example, if you have 100 active users today, you should purchase a 100 user subscription. Suppose that when you renew next year you have 300 active users (200 extra users). When you renew you pay for a 300 user subscription and you also pay the full annual fee for the 200 users that you added during the year.