Upon inviting a user to join your Pipedrive account, that new user will occupy a seat in your Pipedrive account.
Read the difference between a billed seat and a Pipedrive user here.

If you already have an open seat in your account, then that user will occupy that open seat. 
In this case, nothing in the account's billing information will change.

If you do not have an open seat in your account, adding a user will increase the billed seat-count in your subscription.
The increased cost of the subscription will depend on the plan chosen for the subscription, and the amount of time remaining until the next billing date. 

Note: Any credit in your Pipedrive account will be used towards future Pipedrive invoices.