Interest Groups have always, until recently, been a part of TPU. Even from the original DavidKitchin incarnation, TPU has had groups of people interested in specific topics who banded together to form their own sub-groups within TPU with their own admin and web site.
Interest Groups have never really had the partcipation that would have made them great, and thus their success was limited. See TpuHistory for more information on the way in which TeenProgrammersUnite was structured in the past.