Allegiance NPCs accepts declarations of allegiance from guilds wishing to join their faction.
There are two such NPCs, one for each faction in Guild Wars Factions:
To take an oath of allegiance, the guild leader of a guild must reach the capital city of their prospective alliance, which is either House zu Heltzer (Kurzicks) or Cavalon (Luxons). Once there, simply talk to the appropriate NPC above.