DEFCON is the biggest security conference in the world and its impact is so big that some communities started to create local groups to discuss security topics throughout the year, without waiting for DEFCON.

DEFCON is old-school, and is not like the OWASP chapters, which are more formal or academic. It is a totally free community, but not for those who are not serious. You can find DEFCON groups at this link: https://defcongroups.org/.