A reputable agency will not charge you a joining fee. They will interview you and take on your DS if they think they will be able to find work for him. The cost of the initial portfolio photos will be recovered in the commission they take from the payment you will receive when your DS gets work. I would be exceptionally wary of an agency who charges you a joining fee, there are a million scam agencies who will take your money, snap a few photos and you will never hear from them again.