I am a regular listener/reader of the Joystiq Network. I listen to both the X3F Fancast and the Joystiq Podcast and I read their respective blogs. Recently on the Joystiq Podcast the subject of Xbox LIVE grouping came up once again as it often does (Chris Grant will always tell you what is needed by your service or game).
While this is Chris' only, and biggest complaint, for both services (the other one being PSN), it is one that he makes sure to clearly express.
For myself I had not thought about the grouping possibility until I had heard about it from them but now, like the Joystiq staff, I am finding that I cannot go without it more and more.
I would imagine that the system would be built sometime like a Facebook social networking system (we have friends of friends but now we need groups). You could have a Left 4 Dead group, a vegetarian gamers group (my little joke), etc. If you could name the group it would most probably exist. Events could be scheduled by the gamers and once and for all you could be playing with the gamers that wanted to play the game like you wanted to play it (or how it was meant to be played). It would also be a clever idea to be able to set one's preferences to only matchmake within that group (though I am sure that would not be possible by Microsoft's standards because of all of their wacky rules about TruSkill).
The system would probably have a more in depth version based on xbox.com where the moderator could admin events (Best.GamePad.Ever group FTW) and members.
It has come to the point where this is how I want to play all of my games. I have not yet thrown myself to the wolves of matchmaking (in L4D) because I think that I am too horrified of what I might find. Perhaps I would find a preteen zombie hunter that might scream as he hops onto my unwilling corpse. Shudders. That alone might be worse than the Zombie Apocalypse.