Basically they're monthly Hacker related meetings. You'll get hats of every color at them. You'll also get a mix of other things, LockSport, Makers, Hardware people, etc.
2600 is a magazine. The readers have monthly, First Friday, meetings in some areas. In Missouri: St. Louis: Arch Reactor Hacker Space, 2400 South Jefferson Ave.
CitySec is a movement where people in Information Security get together and hang out. ArbSec, BeanSec, NYSec, BaySec etc.
Also you can check out if there are any DefCon(area code) groups. Like DC734 (now ArbSec).
There is also ISSA, InfraGuard, and others if you look for them.
Skill and technique... Google and Research.