Per our standard terms of service, you are billed for all team members (users) marked active during a subscription period.
When a user is added, your method of payment is charged at a pro-rated amount based on the remaining time until the next billing date.
When a user is deactivated during a billing cycle, then a credit for that user is applied to the billing account on a pro-rata basis. A refund is not made to your method of billing.
Note: If you are making a user license switch, it is best to do the deactivation first to avoid increasing your user license count.
Generally, when a team member leaves, you will want to deactivate his or her user account right away to turn off billing. More on deactivating users here.