Longman Dictionary of Contemporary English
special interest group
noun
EXAMPLES FROM CORPUS
▪ Recruiting the practices through a special interest group and through self selection is likely to have biased results.
Wiktionary
special interest group
n. An advocacy group.
Wikipedia
Special Interest Group
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. The term originated on CompuServe, an early online service provider, where SIGs were a section of the service devoted to particular interests.