You will only be charged for the specific time within your billing cycle that the new team member joined the plan.
Monthly Plan - When you add team members to an upgraded library, a pro-rated charge for the remainder of the month will be added to next month's bill. You will then pay this pro-rated charge plus the subsequent month's service bill at next month's billing date.
Annual Plan - When you add team members to an upgraded library, a pro-rated charge for the remainder of time until your billing anniversary will be immediately charged to your payment method.