On Gamerscore Blog, Major Nelson explains the functionality of two new features -- parties and photos -- coming to the new Xbox dashboard (nyah, nyah, didn't say "experience") on Nov. 19. There are more screenshots and a video narrative of the two features at the link.
In a nutshell, parties allow you and a group of up to eight friends to move as one from game to game without reconnecting. The useful scenario given is that of a group that routinely meets up to play Gears of War 2, but someone is always late, so they kill time in Castle Crashers waiting for that guy to arrive. Sounds useful if you do have a Tuesday night Gears league, I guess. But it sounds more like a failsafe for you and a buddy to say, OK I'm bored of this game, let's go play something else.
Photo parties allow you to share photos from a camera or a drive while chatting in real time. Sounds more like a nice to have, rather than a need to have, although I can see it being useful for things like screenshots or maybe sharing annotated maps to help plot strategy. Microsoft's example given, sorry, is simply not plausible: "With Thanksgiving and the holidays coming up, you may even be able to convince your mum and dad to get an Xbox 360 to enjoy all the great new entertainment available through NXE." Uhm, nice try.
NXE: What You Want To Know - Party And Photo Sharing [Gamerscore Blog]