On a Windows, you can add a team member to a project by selecting the project and then entering the person’s email address in the ‘Project Details’ Panel (click the image to enlarge):

By adding a collaborator to this project, you are granting full read and write capabilities. In other words, the project becomes a completely collaborative space. You can see who are the current project members by viewing the list of email address in the Project Details panel (see list underneath the red box for step 2 in the photo above).