First thing, don't speed in the forest preserve. We've had a few individuals get pulled over / ticketed for going like 25 (the limit is 15).
More people show up than the number that RSVP! We usually have at least two games of 5-on-5 going, and almost, almost always have at least 7-on-7. (Winter games are the exception. We still play, but numbers are often weaker.)
We are a pick-up game. Not any sort of championship game. Don't worry about "letting your team down" or anything like that. This is a great way to get outside, get in a good amount of running, and to meet new friends. On Sundays we typically play a little more structured, but it's still the same sort of friendly atmosphere.
All ages and ability levels are welcome ~ we really mean this. Our youngest player was 10 (now 11 or 12), our oldest 69. We have a group of people that play multiple times a week in leagues, and then we have new players come who we teach to play on the field. Some of us are in really good physical shape - others get winded after a few runs up the field. It is a great time and you will fit right in!
The most important thing - this is a pickup game. Our skill levels vary quite a bit. Again, we have league/club/college players and we have completely new folks. If you want to be competitive, match up against someone else playing competitively. If you're a beginner, match up against the other beginners (And ask questions!). We enjoy learning strategy, playing a good game, and getting in a workout - but we are mainly out here to play and have fun.
We distinguish teams by "light" colors (usually white) and "dark" colors (usually everything else). Try to bring a WHITE and a dark (NOT GREY) to each game, so we can make teams even. At the very least, please don't wear grey. It's neither light nor dark and is really good at confusing people in the heat of the game.
The games basically run themselves. So when you come, just jump in. Everyone is really friendly. We were all new to the group once before.