I have been using discourse for years, and am still a bit mystified by group messages. We do not allow members to use it because of this, but do have a helpdesk team who use it.
I just created a test user and added it to the group, and opened the group inbox for two accounts side by side by side on my screen to observe how it works. I noticed two things:
if I archive a message, it will archive it for others in the group. I had thought this was handled individually by each member.
the list of messages in the group is not the same for every member. it seems to be in a different order. I also had thought it would be the same for everyone.
Does it prioritize the messages differently for each member somehow, eg putting messages I am personally in at the top?