After finishing your lvl 10 class quest(Thaumaturge) you can join other guilds. In order to join you have to visit the guild and speak to the receptionist.
For archer you have to travel to Gridania which is a long way through higher level areas so I would advise you to wait and continue your storyline quests until you get the airship pass(~lvl 15) which allows you to easily travel from Ul'dah to Gridania.