What is the process for joining raw agency?

Actually, officially there is nothing as RAW. It is the Research and Analysis Wing of the Cabinet Secretariat. It's higher level officers are drawn from the ranks of police (IPS), defence services as well as allied civil services like the Indian Postal Service and Railway Service. Initially, all officers are on deputation but they can prefer to join the Reasearch and Analysis Service. If they do so, they have to quit their parent service. Lower level officers who form the front field staff (the rank of Section Officers, called Field Officers) are recruited through competitive examinations and interviews. The recruitment advertisements do not come as recruitment for R&AW but as Cabinet Secretariat looking for Field Assistants and Field Officers.