Ambient light is controlled by the shutter speed, the aperture/f-stop and by the ISO. The flash is controlled by the f-stop. By reading the exposure of the background, you would get that exposure then adjust it to match the f/stop of what the exposure of the flash would be while keeping the shutter speed at or under the sync speed. The trick is to make the main subject look as though there is no flash fill, however with a portrait of a band that may not be the case. The flash can create a shadow so the farther away from a background the subject is the less likely you will have to deal with that. Keep in mind that a single flash will only carry approximately 8 to 15 feet depending on its power and what it has to bounce off of and back to your camera. Pray for an overcast day and most of your problems will be solved. Good luck, flash fill is hard to figure out but once you do, it is a piece of cake. You must know shutter speeds, f-stops and ISO and how each one affects each other.
I am a portrait photographer of 45+ years. A watermark, logo, copyright, whatever you want to call it can go on whatever print/image a professional wants to put it on. It is his/her creation and in it is all the years of experience to create it. If you are talking about a paper print you received then it will have the professional photographer's copyrighted logo on it. If you didn't want it on then it should have been pre-negotiated and normaly that would be a higher price, as a portrait without a copyright may be reproduced without the makers consent. There goes his livelyhood and he is out of business. That's why the cost is higher to produce one without the watermark/logo. Even though you are the subject in the creation, the creation is still the property of the creator.