Clan, clique, faction, mafia, pack, organization, camarilla, club, cabal, alliance, band, brotherhood, coalition, family, fellowship, fraternity, gang, guild, league, horde, menagerie, company, corps, community, and union.
Call it what you want. But its close friends that stick together and usually help each other out. Let's say you are dueling and some jerk comes in and is NKing everyone and no one can get their bodies back, plus you lost a lot of gold. If you are in a clan you can call your members for help, and they ususally come. Depends if you are in a good clan or not. Most clans are stupid. They don't understand the support-group feature of the word Brotherhood. Seriously.