After you subscribe to Boomr, you will have a monthly billing date which is set for the same day every month. For example, if you moved from a trial to a paid subscription on December 15th, your next invoice would be January 15th and all future invoices would be scheduled for the 15th of every month thereafter.
On you billing date, you are invoiced for all activity in the previous 30 day period. So if you've added or subtracted users during the previous month, your invoice will show those increases or decreases as a prorated adjustment to your account. You are only billed for users based on the number of days they used Boomr in your billing cycle.
So, if you add a new user half-way into your current billing cycle, you will be charged a prorated amount of 50% of the increase in subscription cost for that user on your next invoice.