Adding new people to pre-established PMs?

Hey @andrew (Our Supreme Lord and Overseer) , I have a suggestion that i'm pretty sure others would appreciate too: A way to add new users to older PMs, so that new ones don't need to be made every time someone wants to join a group chat. Obviously only the owner of the PM should/can be able to do this, but I think this would be an amazing thing to add at some point in the future, if that's at all possible. Thanks for all the hard work you've already put into this site!