I was on a team of 20 (including actors) in NYC a couple of years ago - believe me, everyone had something to do! It was great fun, and something I will do again, but only if I feel up for it - it can be brutal if your ideas are too ambitious. Everyone has to shlep stuff and no one gets any sleep.
There were two Producers on my team, one of them was the lead actor, the other directed. We had a DP, an AC, a sound dude, set designer, two editors, and three PA's. I think there were about five associate producers who all had various duties. Only about four of the team knew each other before - everyone was found through Craigslist and word-of-mouth. There were emails flying for a week or two beforehand - just trying to line up talent in advance (I brought two actors into the project), although we didn't know what types we'd want, it didn't matter. The Producers held a production meeting the day before, so we could all meet and everyone would know who was doing what. We also talked about any equipment or other resources we could contribute, like wardrobe and locations. After the Producers went to the kickoff the next night (we got "Comedy"), we all met again and brainstormed. They took lots of ideas from everyone and put it into the script.
I'm sure the Producer who was also the lead actor had some ideas beforehand about some things - like the fact that she would be playing the main character and maybe it would address relationships or being single, but I know they wrote the entire thing that night. And we had to rewrite it twice after that because we lost our locations during the shoot (neighbors complained), which made certain elements not work without a rewrite.
My jobs as "Associate Producer" was to handle all the release forms, SAG paperwork, crap like that. I also picked up supplies, carried equipment, helped dress the set, found us an alternative location, was asked to be a scripty too (although back then I was clueless about what scripty's do), wrangled umbrellas when it rained on us while we were still shooting on SUNDAY morning, packaged the DVD, went down to the dropoff. There was more, but I can't remember. I know one of the other associate producers went to the store with our set dresser to buy props, don't know what the others did.
Two of the actors and the sound guy made up songs on the spot for it, and that was the only music we used - all original!
Ours was shot on HD with those damn cards that kept filling up and it really slowed us down. They intended to rent bigger cards but could only get smaller ones, so we had to keep sending a PA to dump the cards and free them up. What a pain - for something like this, I think mini-DV is much better!
Our short went on to be a juried selection at a legit film festival about a year later. One of the producers lives in a different state and has done lots of projects since then. His latest 48HFP entry is really, really good and will likely go on to festivals, too.
It's a great experience!