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]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "ingroup":
   Rasputin, Svengali, VIP, access, age group, bad influence, band,
   battalion, bevy, big wheel, body, brigade, bunch, bureaucracy,
   cabal, cadre, camarilla, camp, cast, cell, charmed circle, circle,
   clan, clique, closed circle, cohort, company, complement,
   contingent, corps, coterie, court, covey, crew, crowd, detachment,
   detail, directorate, division, elite, elite group, eminence grise,
   faction, five-percenter, fleet, friend at court, gang,
   good influence, gray eminence, group, grouping, groupment,
   heavyweight, hidden hand, hierarchy, higher echelons, higher-ups,
   in-group, influence, influence peddler, influencer, inner circle,
   junta, junto, key, kingmaker, lobby, lobbyist, lords of creation,
   man of influence, management, manipulator, ministry, mob, movement,
   officialdom, open sesame, out-group, outfit, pack, party,
   peer group, phalanx, platoon, posse, powers that be, prelacy,
   pressure group, regiment, ring, ruling class, ruling classes,
   salon, set, sinister influence, special interests,
   special-interest group, squad, stable, string, team,
   the Establishment, the administration, the authorities,
   the ingroup, the interests, the people upstairs, the power elite,
   the power structure, the top, them, they, top brass, tribe, troop,
   troupe, very important person, we-group, wheeler-dealer, wing,
   wire-puller