Proration of the current month. You signed up and are in a partial month, you are billed a proation for this currrent month. THEN you are charged in advance for the next month. Next months invoice will be what you expect.
On a Family Share Plan, the Primary line will have the highest access fee at which for your line is $110.00 per month and each secondary line is $9.99. You click on the link below for more information on Family Share Plans.