Since there seems to be a new fad of people writing rules for the meeting, I though I would jump on the bandwagon :-)
1. Rules are for wussies.
2. Bring all the toys you can. Got your USRP to do something fancy? Bring it. Find some nifty features in the new kismet? Show those off. Find some new memory corruption in quicktime, but can't quite land code? We can help with that.
3. Talk and listen. Take the amount of time you have been at the current meeting, and divide by the number of people present. This is a length of time denoted by X. X is the ideal time for how much talking you should have done so far. If you have spoken for less than X/2, try randomly blurting out something cool you saw recently. If you have spoken for more than 2X, you should be asking more questions to other people, speaking with more breaks so people can interrupt without feeling like assholes, or maybe you just need to be more concise about what you are saying :-p
4. Don't worry about getting owned. Everyone gets owned eventually, or they live such a cloistered existence that they have no serious understanding of security. People get owned at 2600 because it's funny. No one at 2600 is the type of spiteful bastard that is going to rm your life. Most people there will just laugh and tell you when it happens. There is a rare exception for people who are super annoying, or mega drama queens etc, who might find some rick astley in their live journal.
5. PoC or GTFO. No one cares if you can win the internet in under 10 minutes. Don't give someone crap about running IE6 on win2k if you don't know how to land on it. If you think someone is running insecure tech, then own them. If you think something is easy because you saw someone blog about it somewhere, then try it for yourself. Avoid bragging about being able to do something that you have never done before.
6. 2600 accepts anyone the law allows. Mexicans, bigfoot, hipsters, garbage pail kids, old people, new people, malicious criminals, corporate whores, federal agents, even chan kids. Just show up. It's fun.