Within Teamwork Projects, you can choose to upgrade your subscription at any time, from the Subscription area of your site.
You can see the price plan options in the Subscription area.
To upgrade, click your profile icon in the top right of your site, and select 'Subscription from the dropdown menu:
On the subscription page you will see the plan you are currently on and the various upgrade options.
Each plan has a list of features and you can choose which plan you would like by choosing 'Select' in the plan column:
If you wish to upgrade to the 'Enterprise plan' you can choose to 'Select' and the sales team will be in contact with you shortly.
For more information see: Adding more users to your subscription