After working on a project for a long time, you may have entire cities worth of shapes along with demographic and POI data all specific to the task at hand. When a co-worker wants to continue from where you left off, simply share it with their account. 

You can choose who to share the shape with by clicking on the ellipses to the right of the Project name, and select Share:

You can choose the person within your company to share the project with. If your account has a lot of users, wait for the blue loading bar at the top of the page to disappear letting you know that the user list has loaded in before clicking Select A User.

The person you shared the project with will see it appear in their account and you can continue working safely on the project in your account. Any work you do in your account after sharing will not have an effect the other user's project.

You can also share individual shapes with your company if you don't want to share the entire project.