Everything is a co-forge, it's official. I don't know of any organized lobby that does that. I know occasionally I'll be in a group and they'll throw up something old and released. You could try to make your own lobby that does this though. Its a cool idea, and you know the maps will be good because of all the released ones, the host will like to pick the better ones. But to answer the question, I don't know of any groups the specifically do this.
I don't know of any active lobbies like this. It's easier to put together testing lobbies because we're all forgers and always building something that needs testing. I did run across a lot of customs lobbies in Reach where people would run finished infection, minigame and race maps, but that's not really my thing and I didn't make a point of seeking them out. I seldom ran into customs lobbies for core gametypes (slayer, objective) for whatever reason.
Everything really is a co-forge. I've still yet to not be in one in Halo 4, as every time I try to make something it always either goes down or ends up being the basis for a co-forge. This ends up with a lot of good maps, thought it's weird not having any maps that are "mine" fully. Even then though, in Reach most of the maps I "fully" built had things added in or suggested by tons of different people. FH built Embarcadero.
Just did this, realized I was waaay more relevant in Halo 3 :/ lol. I now know that I have to blow your minds away with something completely new and different. Again.
****IN DO IT Yeah, I've put on the rose-tinted glasses and looked back quite a bit. I did it during Halo 3 when I looked back at my very first creations, at the end of Halo 3 when I saw all my creations from that era, at the beginning of Reach, during Reach (in which I'd often compare H3 and Reach creations), and at the end of Reach. And it's always a very sobering experience.