When do I pay for a user?

You start paying for a user after it has been created in the One.com control panel. Users can only be created by the admin-user, who is the legal owner of the subscription and who will also receive the invoices.

On your next invoice you will be billed for the days the user was active during the past billing month.

Related articles:

How do I create a new Office 365 user?
Can I try Office 365 for free?