By default, when you subscribe to a paid plan your billing will be set to automatically adjust when new users are added to the account. Because Boomr is pay-per-seat, this allows you to invite new employees to track time without being required to make a manual adjustment to your subscription.
If you would prefer to turn this feature off you, may do so at any time by going to the Billing page and selecting Adjust Plan. From here, simply unselect the checkbox in the bottom right corner. Once turned off, you will not be able to add additional users to your account until you update your subscription to allow for more users.