Backstage has now added a new option to invite collaborators to a project.
Begin by clicking on the "manage collaborators" link:
Then click on the "invite a collaborator" option that appears:
A window will pop-up to select the production wishing to invite the collaborator as well as to send the collaborator a message:
It's also possible to remove a collaborator from a project by clicking on "unassign" next to the name of the collaborator:
If the person is the collaborator on a project, a collaborator icon will appear next to the title of the production: