This use names the organization, not a person. English says it with the in front and with a plural: join the Cub Scouts, be in the Cub Scouts. Younger children start here, and when they are old enough they move up into the Scouts. What happens inside is camping, knots, small skills, simple service jobs, and badges for each skill learned. When the word is naming one child instead, the article changes: a Cub Scout.

Here the word names a person: one child in the junior part of Scouting, usually of primary-school age. Cub Scouts have a uniform, meet in small groups, and collect badges for skills they learn. In a sentence this sense is countable: a Cub Scout, two Cub Scouts, my brother is a Cub Scout. That is the quickest way to tell it from the other sense, which names the organization itself and takes the in front of a plural.