A special interest group (SIG) is a community within a larger organization with a shared interest in advancing a specific area of knowledge, learning or technology where members cooperate to affect or to produce solutions within their particular field, and may communicate, meet, and organize conferences.
All interest groups share a desire to affect government/institutional policy to benefit themselves or their causes. Their goal could be a policy that exclusively benefits group members or one segment of people (e.g., better documentation practies) or a policy that advances a broader public purpose (e.g., improving life quality).