The Refined Training feature Default Group allows you to specify a default group for a course as well as a default grouping.
Once groups and groupings are set up, you can specify defaults for the course. Go to Administration>>> Course administration>>> Edit settings. Click on Expand all and scroll down to Groups. Use the drop-down menu to select the Default Group that users will automatically be assigned to when they are enrolled into the course:
Setting a Default Group will designate that value as the default group for all future enrollments into the course unless otherwise designated in the enrollment process (such as in API calls).