I think the most we managed is six people: trumpet, coronet, two trombones, euphonium, and a tuba. The trumpet was the driver, and nominally only played when we were off the main streets (but was good enough to play off-hand and still drive); coronet in the center front, trombone shotgun (right front), tuba back right, euphonium (me) back left, and the center back trombone over someone's shoulder depending. This was a fairly comfortable arrangement, but sometimes we'd swap me and my euphonium into the shotgun seat and put the two trombones in the center front and back, with slides out the right windows, to get more "broadside" power projecting out the sidewalk side of the car.
Sometimes we'd drive down main street and around town, caroling madly with the occasional fight song or high-energy classical piece thrown in for variety. Other times, especially on weekend nights after dark, we'd cruise silently up to some business, preferably a bar, club, or theater with a big line of people outside, with the instruments pulled inside as much as possible; then on a count of three shining brass and gleaming silver would erupt from every window and they'd be hit with "God Rest Ye Merry, Gentlemen", "Night on Bald Mountain", or some other powerhouse piece from point-blank range.
We were never bothered by the cops; we made sure that the driver had both hands on the wheel when we were on a real street (as opposed to a movie theater parking lot, for instance). As far as we could tell, the town had never considered drive-by brassing when writing ordinances, so there were no rules against playing an instrument from a moving vehicle on the books :) And we kept mostly to Christmas carols, although admittedly primarily high-energy ones, while in residential areas; we cleverly figured that anyone complaining about "a bunch of kids caroling" in their neighborhood would be laughed off as a Scrooge and ignored.
We did consider doing Halloween caroling one year, but sadly never got around to pulling it off.
The other group I've brassed with was an ad-hoc group with me and my euphonium and three tubas in the back of a large van on the way back from a huge regional Tuba Christmas event in DC. It was interstate for most of the way, but we played some as we pulled out and a bit once we got back in to town. We were already pretty brassed out, though; several hundred tubas and related low brass on the Mall under the lights was a real experience.
Seeing the community has really brought back the memories... I think I need to put on a Canadian Brass CD and plot how many of my friends still have instruments in their closets...