The caste of Thieves is recruiting in Port Kar. There is no "head of caste", that role is taken by an NPC. Anyone can show up and roleplay a thief (casted or not) we will not try to police your roleplay.
However, if you are interested in joining up with other Casted Thieves then simply contact me, Kafka Khaos in world and I will hook you up.
I also have free "caste of thieves" face tattoos that I can give you to complete your look.