Why can't I add a Group to a Step-based Course?

Since Step-based Courses are by nature self-paced, this runs contrary to the idea of a Group Course, which is for multiple clients to progress through the Course material in lockstep. Thus, Groups can only be added to day-based Courses.

You can, however, select multiple clients and add them to the Course all at once by selecting "Actually I want to add several participants..." when adding a participant.