Monsieur Cerulean,
There are many Guilds in Andar. A Guild is simply a government-recognized organization dealing with the administration of a specific subject. The Earth Guild covers most facets dealing with Earth magic, including resurrections, healing, the physical wellbeing of Andar's citizens, and in certain circumstances deals most directly with stamping out necromancy.
The Brotherhood of the Star is, I believe, the local arm of the Andarian Celestial Guild by charter. The Guildmaster, Shikar al'Basteua, can correct me if I'm wrong. They deal with identification of magical items, which is the major power of a Greater Celestial Circle. To put up such a circle requires license from the Andarian government.
Around the Crossroads and Andar in general are many other Guilds or Guild-like organizations. For instance, the Magistratum is an organization which has a license from the Andarian government to conduct certain studies (and has license to have long-term Circles of Power). I do not know if they are specifically chartered as a "Guild", per se. There are several trading and mercantile Guilds as well.
It would not be wise for one Guild to "oppose" another, as all true Guilds are chartered by the government. There are many Guild-like organizations that are not specifically chartered, however, and those might oppose various other factions depending on their aims at the moment. In those specific cases you should talk to the individuals in charge of said organizations.
-Polare Lissenstine
Seneschal, Count Di'Orsini
