I keep being confused by references Bulk Add to Group, since it doesn’t appear in the UI as far as I can tell—the screenshots from the 2015 topic you linked to don’t match with what I see now. Is it just the same as using the Add Members dialog within a group?
And how can I copy a list of members of one group to be able to bulk add to another? The best workflow I can come up with at the moment is to copy the text of the Members page and use a style-savvy grep (in Nisus Writer Pro) to copy the usernames. Doable, but not exactly easy.