There are a number of great security organizations in Colorado.
The Denver chapter of ISSA is the largest chapter in the world, and a lively place to meet with local security professionals. ISSA is a great choice for security professionals. The chapter holds monthly educational lunch meetings, regular full-day training events for members, half day industry vertical events, and happy hour socials.
The CTA is focused on advancing the technology industry throughout Colorado. While not focused specifically on security, it overlaps with the security community in a number of places. It it a great choice for those who want a broader view of the IT climate in the region.
The Colorado Springs ISSA chapter provides educational and networking opportunities to security professionals in the Colorado Springs area. Due to it's location and the heavy government influence nearby, it is a great choice for those interested in discussing security in the government or defense contractor industries.
ISACA is an international organization that advocates for information security, assurance, risk management and governance. ISACA is a great choice for those interested in the audit and compliance world. The Denver chapter is one of the biggest in the world, and has won their international chapter of the year numerous times. They hold monthly lunch trainings and social events.
The Open Web Application Security Project (OWASP) group is an international group focused on improving software security. Their Denver chapter is a great choice for those interested in the software security side of things. They generally meet every other month for dinner meetings and trainings.
City Sec is an informal group of security professionals who get together on a bi-weekly basis for happy hour to network and talk tech. Follow them on Twitter for meet up details. City Sec is a great choice for those looking for a laid back chance to meet with brilliant security minds to talk shop.
Colorado Cyber exists to raise the profile of Colorado's information security ecosystem. Since it's creation in 2015 is has worked primarily with local security vendors and entrepreneurs to make Colorado a more hospitable place for security companies. It is a great choice for those looking to be more involved with the business side of security in Colorado, and help influence the political climate here.
The Cloud Security Alliance (CSA) is a group focused on providing resources for securing data in the cloud. This group is a fantastic fit for anyone who works at a company looking to move into public, private or hybrid cloud environments.
InfraGard is a partnership between the FBI and the private sector, to facilitate sharing of important data and prevent attacks against the US infrastructure. The Denver chapter has regular meetings on a variety of topics, including information security, physical security, business continuity and more. Access is only provided to those who have been through the FBI's vetting process.