This is something I've been meaning to do for some time. Customs Night is coming to DBO (or at least the sign-ups are). If you don't know what Customs Night is (which is more likely than the last couple dozen times I've typed that), I'll let you know now: it's a get-together held every Friday night by Community Customs (which was recently reset) in Halo where we play (are you ready for the surprise?) custom games. We play custom and standard gametypes on custom maps (and occasionally a standard map or two), with the maps ranging from tiny to enormous and the maps and gametypes ranging from pretty competitive to standard to completely silly. Specifically, we start playing roughly at 9:00 PM Eastern time, and this week we'll be playing Halo 4. We pretty much play the maps and gametypes people suggest, although I try to separate things from the same person if someone else has things they want to play, and sometimes someone asks to have a map tested but can't be there, and I usually make that happen provided there are enough people.
For the more visual-minded, you can expect things like this:

and this:

and this:

actually, I'm pretty sure that's not from Customs Night, I should've used something like this:

er, that's Halo but it's not from Customs Night or even the right mode... more like this:

no, that's how you'll feel at Customs Night... maybe a video will work better, one like this:
https://dl.dropboxusercontent.com/u/4209180/Halo/video%20%28small%29/long%20stick.wmv
or this:
https://dl.dropboxusercontent.com/u/4209180/Halo/video%20%28small%29/Watch%20for%20falling%20fails.wmv
(if you think a lot of that is from Reach, you're right, I haven't got that much stuff from Halo 4 yet; if you think a lot of that is negative, you're largely also right; to me, things that are a little negative tend to be more funny than other things, and so have more tendency for me to save them)
If you think that sounds like fun, you can join in by just replying to this post and saying so and giving your gamertag. If you do, you'll get a message or an invite when we're starting (the exact time and whether it's an invite or a message depends on various things).
As far as rules or regulations or guidelines, aside from not being a jerk the only things to know are that a Community Customs admin-- a group including but not limited to nomis78, ShadowoftheVoid, and myself, GT: Gnrl Vagueness-- will have party lead the whole time (for reasons that should be pretty easy to figure out and understand) and we won't run any modded content (because Microsoft could easily ban people for having it or even playing it with no warning).
Community Customs started out as a revived HBO Customs, then dropped that title for Customs Clan, and changed again to Community Customs somewhere around two years ago (I think, my memory isn't that good), and we've been through several domain names, several web hosts, and several (or more than several) hosting applications. I've been playing in Customs Night as long as it's been around, and helping to run things for a year and a few months now.