Dr wants to do a bone graft surgery from my gum to lift sinus membrane. I wear invisalign retainer daily, is implant necessary? Cost vs risks?

Cannot. Cannot make that decision for you. If tooth needs to be replaced that's a better solution than depending on retainer to hold space. If not enough bone for implant/crown a sinus lift may be the appropriate route of treatment. You need to be examined to answer your question, and we can't do physical/x-ray exam here. If your not sure, seek a second opinion.
Sinus augmentation. To make room for the bone, the sinus membrane has to be moved upward, or "lifted", sometimes also called a sinus augmentation. It is done when there is not enough bone in the upper jaw, or the sinuses are too close to the jaw, for dental implants to be placed. The benefit is a dental implant. The main risk is that the sinus membrane could be punctured or torn. It is also very easy to repair. Best.