Contact Group is a social, mundane advantage, described in Basic Set, p. 44, and referenced in GURPS Dungeon Fantasy 9: Summoners™, p. 18, which adds the specialization Contact Group (Local Spirits).
Sean Punch has commented on Contact Group in the following posts to the SJG GURPS Forums:
- On replacing Contact Group with Rank in GURPS Action 
- On high effective skill levels, and whether Contact Group or Allies is suitable for a character 
- On a Contact Group actually being an individual , 
- On its geographic coverage