The longer answer is that they log in the same place that the site admin does, typically www.[siteURL].com/wp-login.php Going to the dashboard URL directly (www.[siteURL].com/wp-admin) will also redirect to the login URL if that is easier for them to bookmark.

User roles don’t have separate login pages or back end interfaces. They just have a limited set of features in the standard WordPress backend Dashboard.

Does that make sense?