When you upgrade to a paid plan you will be billed for the number of users that are currently members of your upgraded libraries. You'll have an opportunity to preview the exact charge prior to upgrading.
You will be subsequently billed at your next bill date (a month or year from now). This bill will cover the subsequent month or year of service. If you add team members, you will be billed immediately for an annual plan or in your next bill for a monthly plan.