First off if this is old, I apologize as I used the search function and found nothing.
You can in fact join the guilds in this game.
The requirement is that you reach Rank 20 of the Class for the Guild you want to join.
Travel to said Guild and speak with one of the masters behind the desk, from there just read and answer the prompts.
I just joined the Armorers & Blacksmith Guild in Limsa.
They also gave me a linkpearl and I am currently doing my first quest!