There are many professional organizations with an interest in usability. Alongside the UPA (creators of this site) you can also find your peers in the following areas:
Usability Professionals' Association International
UPA supports people who research, design, and evaluate the user experience of products and services. Many local chapters around the world.
Human Factors and Ergonomics Society
Theory, principles, data, and other methods applied to design to optimize human well-being and overall system performance
Association for Computing Machinery Special Interest Group on Computer-Human Interaction
Human technology and human-computer interaction (HCI)
AIS Special Interest Group on Human-Computer Interaction
Human-computer interaction (HCI) for business information systems
The British HCI group
Analysis, design, implementation and evaluation of technologies for human use
The Ergonomics Society
Application of scientific information concerning humans to the design of objects, systems and environment for human use.
International Ergonomics Association
Federation of ergonomics and human factors societies
Internet Technical Group
Internet technologies and related behavioral phenomena
Society for Technical Communication
Advancing the arts and sciences of technical communication
STC Usability and User Experience
Usability and usability assessment of technical communication
STC Information Design and Architecture SIG
Application of design principles to translating complex, unorganized, or unstructured data into valuable, meaningful information, combining skills in areas including graphic design, writing and editing, instructional design, human performance technology, and human factors.
Professional Communication Society of IEEE
Understanding and promoting effective communication in engineering, scientific, and other technical environments
ACM Special Interest Group for Documentation
Teacher, researcher, or practitioner of technical communication, computer science, user interface design
AIGA Experience Design
Organizational connection and communication, understanding people, in ways useful for business and design, and conceive, envision, and inform what products, services, and communications to make.
Interaction Design Association
Defines the structure and behaviors of interactive products and services and user interactions with those products and services
The Information Architecture Institute
The art and science of organizing and labeling web sites, intranets, online communities, software, and hardware controls to support usability and findability.
Association for Computing Machinery special interest group on computer graphics and interactive techniques
Promote the generation and dissemination of information on computer graphics and interactive techniques
International Institute for Information Design
Design of visual information
American Society for Information Science & Technology (ASIS&T) Special Interest Group Information Needs Seeking and Use (USE)
People's behavioral and cognitive activities as well as their affective [many people don’t know what this means – should we use a synonym (emotional) ?] states as they interact with information
American Society for Information Science and Technology Special Interest Group Information Architecture
Art, science, and business of organizing information so that it makes sense to people who use it
American Institute of Graphic Arts
Includes book and type design, communication design, interaction design, experience design and motion graphics.
Design Management Institute
To advocate the economic and cultural importance of design, and to make accessible a body of knowledge on design effectiveness in business
Industrial Designers Society of America
Creating and developing concepts and specifications that optimize the function, value and appearance of products and systems for the mutual benefit of both user and manufacturer
International Society for Performance Improvement
Improving productivity and performance in the workplace
American Society for Training and Development
Workplace learning and performance professionals
Association for Computing Machinery
Advancing the skills of information technology professionals and students worldwide
The Institute of Electrical and Electronics Engineers Computer Society
Provider of technical information, community services, and personalized services to the world's computing professionals
Project Management Institute
Project management professionals worldwide
American Society for Quality
Advances learning, quality improvement, and knowledge exchange to improve business results, and to create better workplaces and communities worldwide
International Federation for Information Processing
Umbrella organization for national societies working in the field of information processing.
American Society for Information Science & Technology
New and better theories, techniques, and technologies to improve access to information
Marketing Research Association
Your link to a world of powerful resources
The Product Development and Management Association
Resources for professional development, information, collaboration and promotion of new product development and management.
American Psychological Association -
Professional and scientific organization for psychologists in the USA. [note that there is probably an international organization but I’m not up on that.]
APA Division of Applied and Experimental and Engineering Psychology
The division of APA that focuses on applied experimental and engineering psychology research to improve the ability of humans to operate more effectively in a technological society.
Computer Professionals for Social Responsibility
A public interest group that is concerned about the impact of technology on people.