Restrict replies to a certain group of users

@merefield I’d have 4 categories
Category A
Cateogry B
Category C
Category D

within those categories there would be several subcategories.

user group 1 can only create topics and reply to their OWN topics.
User group 2 can reply to any topic and create topics.

this is not possible without a plugin? do you think a plugin could be made for this if its not possible?