One user corresponds to one mailbox of 5, 10, 15 or 25 GB based on the base plan you choose. However, a user can have many email or domain aliases (ie. different email addresses), with all emails accumulated in the exclusive mailbox assigned to the user.
The Free plan comes with 25 free users or mailboxes of 5GB storage each. When you are in need of more than 25 users, you'll have to upgrade to a paid plan wherein, all user mailboxes will be upgraded to the higher, paid plan storage along with additional features. So, you will have to pay for all users, including the 25 existing users.
Your account will be downgraded to the Free plan with 25 active email users. The other user accounts (all but the first 25 mailboxes created) will be deactivated. The email data will be available for export for a limited time period.