Hi,
I think the change about kicking is good, messages not disappearing just because you got removed from a discussion makes sense.
However, this with inviting I don't find so necessary, and in fact I would think that it should be optional if you want the previous history of a discussion to be hidden, rather than make it hidden by default.
If I invite someone to a discussion, it's probably because I want them to take part in it/collaborate on something, so it doesn't make sense that I have to re explain the entire discussion now again after this person joins. It also doesn't make too much sense if the person just received a discussion with a specific topic, but the only message in it for them is a message from Admin telling them that they have been invited.
Taking another example, maybe I am sending scores to a tournament organizer via permanent messages, and I forget to include one by mistake after sending the message. The logical thing to do is to invite them to the discussion, but now it doesn't make any difference because they cannot see the first message, thus somebody else has to re-post the same message.
I hope that makes sense and you can reconsider this feature. I personally wouldn't mind it too if I had to first enable an option before other messages of a discussion can be visible to newly invited participants, but I think it should definitely exist if the plan is to definitely make this change.
Thanks for explaining how it's supposed to work.