It is also possible to “reply as new group message” - see the arrow on the top left corner of the composer. This allows you to create a new message starting with the same recipients. You can then adjust the recipient list without having to explicitly invite them to the message or give them access to the past discussions.
This is especially interesting for folks using group inboxes to provide email support, with group inboxes. Sometimes you want to expand the conversation to include more email addresses, and this is one way to do it. Discourse for Teams is well suited for this, as per: https://support.teams.discourse.com/docs?topic=106