When you invite a user to join your Pipedrive account, that new user will occupy a seat in your Pipedrive account. If you would like to learn more about the difference between a billed seat and a Pipedrive user, click here.
For example, if your monthly Advanced plan ($29 per seat, per month) billing cycle started on November 1st, and you add a second seat on November 15th, you will be charged $14.50 on November 15th, for the two weeks left of the current cycle for your second seat.
On December 1st, you will be billed the total amount ($29 x 2 = $58) for two users on the monthly Advanced plan.
For more information on pricing, take a look at our pricing page.