Hi @WayneLee,
I agree that group management can be a bit of a pain to manage. For our faculty, I almost always suggest waiting to create groups in Canvas until the end of what we call our "add/drop period", where students should not be able to freely make changes to their classes anymore. Waiting until a roster is pretty stable (if possible) will avoid a bulk of the group enrollment issues you mentioned.
As for the group discussions, if you want each group to respond to the same prompts, all you need to do is create the discussion in the main course shell, and select the "group discussion" option (guide at How do I create a group discussion in a course? - Instructure Community). What that does is essentially put a copy of the discussion prompt in each of the group shells, so the students all just reply in their group.
Hope this helps!
-Chris