The Cairhien guild is a somewhat different entity than many of the others. It represents the court and strong-arm of the Cairhien government. Thus, the members of this guild (which is of course headed by Laman) include notable members of the various noble houses, as well as city watch staff.
It is more a support and gathering mechanism than a guild that functions on the same political level as any of the others. Membership in the Cairhien guild is often times simply automatic on advancement in another group to sufficient status. However, players wanting a role in the city watch, etc. should contact Loren, or request an appropriate SkillSet from the Immortal Staff.