Is there a way to create subgroups from different classes?

Khan Academy allows you to have the same student in multiple classes. I would suggest adding an "intervention" class with all of those students in it. Please let me know if you have questions or need clarification.

So the "intervention" class would have its own class code for the students to enter? The activity/progress they make will show up in both their "regular" class and the "intervention" class?

Yes, the intervention class would have it's own class code. However, students would not be required to accept you as their coach again since they have already done that for your regular class. Activity/progress will show up in both places.