an option can be to have this type od task already defined in your workflow with the necessary action and instruct users to insert that task from the template vs. a completely new.
e.g . if the task for group B/C is the same that the one from group A
you will add notify assignee/group action to the intial task used for group A and instruct your user when inserting new tasks to select this template from category ... and define assignee/group B/C at the same time.
I think any others will require customization
Hope this help.
/J