I got inspired by MoonAstraea's post (and past complaints from others) to make a ticket about this.
I really think this would be a helpful feature to prevent future frustration.

Let's say you are added to a group chat that you didn't want to join and there's no way to leave.
I think we've all been there, and it's aggravating because even though you can delete the pm, it still comes back if someone sends a message.

So what if there was a confirmation step where you can accept or deny an invitation to join a group pm? Similar to Discord?

The person who's making the chat can do the normal set up where they add the users they'd like and write the subject and greeting message. So it would work just like normal.
But before you can officially open the pm, it will have a confirmation. "(user) has invited you to a group private message with (list other user(s) here)! Would you like to accept?"
The subject of the pm would be shown, and it could show you a preview of what the person's message is.

And so, if you choose accept, you will be forwarded to the pm.
If you choose decline, it forwards you back to the inbox and trashes the pm for you.

The overall idea applies to private messages with more than two people, but it could be nice for the messaging system as a whole.

But yeah! That's basically the gist of it. :)
If anyone else has ideas to go along with this, I'd love to hear it!