The Team Builder, or Stage Resourcing, can be accessed in a couple of ways. These methods are available both on the Project Planning board, and on the Project Schedule (Enterprise only).
From the Project Planning board:
Click on the arrow to expand a project
Click on the pill for a stage. This will open the side panel for that stage.
Click on the Stage Resourcing icon.
It can also be opened by clicking on the + icon to add a resource under a stage. Either hover your cursor over the resources to see the icon, or expand the resources:
From the Project Schedule (Enterprise only):
Accessing the team builder from the project schedule is similar to the project planning board, without the need to expand the project first.
Click on the pill for a stage. This will open the side panel for that stage.
Click on the Stage Resourcing icon.
You can also open it from the resources below the stage.
Hover your cursor over the stage resources to reveal the + icon, or click the arrow to expand the resources.
Click the + icon to add a new resource.
