How do I pull a field value from a form field in to my inline entity form?
Just want to share my solution to this problem. Turns out I didn't need another function, I had all the variables available in my MYMODULE_inline_entity_form_entity_form_alter function in the $form_state array.

I targeted: $form_state['input']['field_group_reference'][und]

And that got me the value of the field in my parent entity.