Are Collaborators using the URL
to access the editor?
In that case there should be a login bar at the bottom of the viewport.
To be clear, the Homepage opening from the Dashboard when you click on Editor is expected behavior. You can then directly make changes in the page or use the bottom bar to access the CMS backend.
This has to be done in Chrome or Safari only. Not only for the developer (you) but also for Collaborators. (the ones you define in the Dashboard)
The collaborators also access the site directly, they see the homepage, not a dedicated Editor login page. They can log in/out or manage content using the bottom bar.
The Editor bottom bar can be very discreet because it can collapse and often loads collapsed. Look for the little arrow tab on the bottom right.