Most versions of the LNH have a huge, diverse membership. A subgroup is how members of the Legion organize themselves into smaller, friendlier units that are easier to work with and within.
LNH subgroups are semi-independent, usually going on missions together. They are often built around a theme or tasked to deal with a single area. The primary cast of a single series often belong to a subgroup. Members of a subgroup may all be creations of the same Writer, or may be gathered together from older, Not Reserved and Free For Use characters.