There are two ways in which you can add staff to your project.
The easiest way to add project staff to your project if you have just logged in, is on the 'Project Home' page. This is the first page you will see when you log in. You will see in the main text a green button, 'Assign A User.' By clicking on this button you will visit the 'Team Members' page. In the top right of the screen is a green button '+ Add New Team Member.' By clicking on this button you can invite people into your project by adding their email address and conveniently assign a user role at the same time.
Alternatively, you can go the the Team Members page at any time from any page by clicking on the 'Team Members' link in the left hand side navigation menu.