It’s simple! Just click on a group that you want to add a subgroup to.
Once that’s done, click + Create a Sub Group at the top right.
Now simply name the new sub group, and click Add Sub Group. If you want the sub group to have the same enrollment as the parent group, check Inherit courses from parent group before you click Add Sub Group.
To delete a sub group, click on the one you want to delete, navigate to the Settings tab and click Delete at the bottom.