The Aiel guild is one representing a whole people. All the people of the Aiel Waste gain automatic membership in this guild, in accordance with the fact that the only way to be an Aiel is to apply for one.
That being said, there isn't much else to write. The guild serves as just one of the ties that bind the people together, though we don't currently have any people playing Aiel.
Please note however, Aiel are application only characters (so-called SkillSet or simply SSet characters). Thus, to obtain an Aiel, and join the Aiel guild, you have to submit an application via the proper channels. In addition to the typical application requirements however, the application should include a detailed essay on the type of Aiel for which one is applying (clan, society, etc. whatever the facets of the character). For more information, contact a member of the Immortal Staff.