The Collaborative International Dictionary of English v.0.48:

in-group \in"-group`\, in group \in" group`\n.
   an exclusive circle of people with a common purpose,
   interests, or attitudes, especially one that produces
   feelings of camaraderie, exclusivity, community, and
   solidarity.

   Syn: clique, coterie, ingroup, inner circle, pack, camp.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

ingroup
    n 1: an exclusive circle of people with a common purpose [syn:
         clique, coterie, ingroup, inner circle, pack,
         camp]