Within Teamwork Projects, you can enable tags on your site, and also specify whether everyone, or only administrators, can add new tags.
To do this, click your profile icon in the top right of your site, and select Settings from the dropdown menu.
In the Settings area, go to the General tab.
Here, you will see an option to Allow tags to be used on your site. You also have an option to lock the editing of tags to administrators only.
When you enable the tags option, you will also see a Tags area in the site settings menu for managing tags.
For more information, see: Tag options