Hi, Joanne!
At this time, we are unable to track training completions on Workfront One, as the website doesn't offer the same features/functionalities as Workfront Ascent (which is where our Workfront Classic training is accessed). It is something on our training roadmap.
Therefore, Workfront One logins aren't needed unless someone wants to participate in Community or if they're the assigned support contact for your organization.
I've seen customers track training a few different ways, using the "honor system." One customer had an onboarding project for each user, with each training course they needed to take listed as a task (with the URL in the URL and/or description field for easy access). Then the user could just complete the task when they were done with the training. Other customers have used a request queue that users submitted to when they completed a course.