The various guilds of Zenith have different requirements for membership.
You will find that it can be as easy as asking or as difficult as a month long trial run depending on where you apply.
You can join 1 guild per character, and *only* the guild leader at present can invite you to the guild in game.
There are many maaaaany guilds out there.
If you follow the link you will find a small list of guilds out there.
Please know this list is far from complete. It will, however, give you a good idea of what types of guilds are out there for you. :)