The Project Management platform works seamlessly with our standard and enterprise platforms, but is sold as a separate subscription.
To add access to the Project Management platform, just go to Settings > Team and select the team member that you'd like to add PM for.
You will see an overlay with that user's permissions. Select one or both of the permissions at the bottom under the "Project Management" heading.
Note that selecting these options will automatically initiate a payment for Project Management. (If you are in the middle of your billing cycle, the payment will be pro-rated.)
To remove a PM subscription, just deselect the boxes under the "Project Management" heading.