A circle pit is a particular kind of mosh pit where you create a large circle, often at the request of the band on stage, and then all the participants move around the circle, flailing their arms and jumping around and such. As a rule, most people move counterclockwise, so that their right arm is keeping the circle nice and structured. As a rule, it is also wise to move in whatever direction everyone else is moving, lest you desire a whack on the side of the head. To properly join in a circle pit, you should know a few things:
  1. Keep Going: Move in the same direction, same speed as everyone else. Disruption of someone elses hard won circle pit (as circle pits are only given out as trophies for the most difficult of contests)* can result in a good deal of anger which, when mixed with a lot of adrenaline, can result in more and more windmill dancing in your direction.
  2. Don't Mosh: Dont run across and start ramming into people. Many circle pits look like mosh pits in the beginning, so make sure you know which you are getting into. A little arm swinging and small kicking is usually allowed, depending on the atmosphere. Just check out what everyone else is doing, and emulate.
  3. Don't Pull People In: If someone wants to join in, they will. Pulling people in makes you look like a dick, makes the other person scared shitless, and can disrupt the flow of the pit.
  4. Don't Be a Fucking Pervert: Believe it or not, there are some people who would disagree with me on this. I say fuck them. The pit is not a place for you to start grabbing girls' tits and crotches, or start rubbing your crotch on people. Sure, the pit can be a great way to release tension, but sexual tension should be released at home, into a tissue. The amount of girls that can kick your ass in the pit is rising, so I recommend that all the perverts out there control their urges (before someone decides to control them with a steel-toed boot).

Oddly enough, I found this an empty nodeshell.
*circle pits are really hard to start by people in the crowd, so don't fuck them up.