When you go to a clan's title page and choose the postulate option, it lets you apply for membership to the guild. When the guild's admins see your application, they can choose to accept or decline. If they accept, you gain membership into the guild you applied for.
It's true there's so many guilds out there for ya to join. Might as well join one with a cool name, like oh, this one : http://www.urban-rivals.com/guild.php?id_guild=2114