"Standard" accounts are free and can have up to 10 users.
"Professional" accounts can have unlimited users and have access to additional features that "Standard" accounts cannot access:
Professional accounts are charged a base fee of $49 per month, paid in advance. In addition, fees are charged monthly based on the activity of your account. Up to three projects may be marked as Sold (i.e. the Sold milestone is marked as completed) with no additional charge. From 4 to 15 projects marked as Sold per month, you will be charged $49 per project; from 16 to 30 projects marked as Sold per month, you will be charged $44 per project; and, for more than 30 projects sold per month, you will be charged $39 per project.
Click here for more information about the Professional Edition.