What occupational group is not likely to have a large representation in an interest group?
Every large occupational group has a lobbying group in Washington, and the smaller groups (carpenters? auto mechanics? plumbers?) can join groups such as the Chamber of Commerce. There are also indirect interest groups; while the branches of the Armed Forces do not have their own interest group, it could be said they are represented by the large contractors who sell weapons systems, warships, aircraft, and similar, to the Dept of Defense.