I am unable to add a user to a project role unless they are already linked to at least one project role.

I am attempting to do this via Administration > General > Users and then select the "Project Roles" link for that user.

If there is already a project role listed for the user, then the "Edit Project Roles" button leads to a screen where there is a 'Display Project' link.

This link allows additional projects to be added to the list. However, if there is no project role listed, then the "Edit Project Roles for User" screen does not have the 'Display Project' link.

In this case, the only way to add the user to the project appears to be from the Administration Dashboard for the specific project, which is not very effective if trying to manage adding a user to roles across multiple projects.