The pay-per-use pricing plan charges for the total meeting minutes consumed. For example, if you have a meeting with 5 people for 1 hour, the total meeting minutes would be 300 minutes.
With the monthly plans pro100 & pro500 your company account is allocated 5000 -or- 20000 minutes respectively. All additional minutes are charged at the reduced rate per minute.
Once activating your account, as an admin, you can add users under "members" - "add user". Alernatively, you can allow users at your company to activate their own account using the custom sign-up form.