What do you mean by "a person's option to not be included"?
Does that mean if I don't want to be included in a PM-Group then I just click a button? Also to avoid any major troll abuse of this if it gets put in I think allowing only one invintation to any member at a time and the only way to invite them again is if they decline the first invite.
Maybe you can also click a button to 'hide' an invite from somebody so if you get invited by a troll or a group constantly when you don't want to join it then you can click 'hide' on the invite and it goes into a subfolder of sorts in My Stuff: PM's.
We could set a "Make PM Group" button above the "Send Message" one so it's easy to find as well.