i want to be able to link a Subitem to a group allowing for the group to contain all the tasks for a subitem, much like ths diagram attached
You probably need to get more specific. Describe a single item flowing through as you imagine.
Using the image above, I would want to have it so that when all the sub items in the DD Design Meeting 1 group are completed then the sub item in the Project Schedule marks as complete.
I think the beginning of your solution is below. Let me know if you need any help.