Within Teamwork Projects you can add users to your account via per user pricing plans in amounts corresponding to your actual team size.
You can choose how many users you require in your site subscription settings.
Click your profile image in the top right corner of your site, and select 'Subscription' from the dropdown:
You will then have an option to update an existing subscription or create on if you are currently on trial.
You can choose to add more by using the per user slider or entering the exact amount into the team size box:
Note: When adding users for a paid subscription you will be required to choose a minimum of 5 users. If you are on the free forever plan you will have a maximum of 5 users.
For more information: Adding more users to your subscription on per-user plans