You can add and remove users if you are an administrator for your organization.
When adding enough users to your organization that you change the size of your plan, you will be prorated the difference for the remainder of the payment period.
For example: if you are on the 5 user plan and add your sixth member with one week to go on your payment period, you will be charged for the additional five users to reach the 10 user plan for the remaining week. In the next payment period, you will be charged for 10 users the whole month or year depending on the plan you are on.
Please be aware of adding and inviting members and keep track of this in your organization. If you add members and go over your bucket size, you will be charged.
When removing users, leading to a downgrade of plan, you are given a credit for the time left on that payment period, not a refund. This credit is then used when upgrading again or during your next renewal before charging any additional costs.