I can't see any Group Grading options when I try to Grade a Lesson in Lt

The Group Grading Feature was released and implemented within every Lt installation on 13th June 2016. Only Courses created after this date will have the Group Grading Feature available.
Any Courses that were created prior to this date will retain the older Grading Interface, without Group Grading functionality. To add this functionality, create a new Course and copy the Modules / Lessons from your old Course to this new Course.
Note that Student Accounts and Data on that Course are not transferred by doing this. Exercise caution if you decide to delete this old Course!