Outdoors, you're going to have to keep the shutter speed at sync speed or below. That and film speed will determine your aperture. If it's a small aperture, your portable flash may be fine for individuals and couples and tight shots of small families, but not powerful enough for big group photos. The umbrella will drop your output a bit depending on the style. You can put the flash closer than you are, but just make sure it's not in the photo (not a problem with tele lenses) A fabric reflector kit can also be a good alternative, but sometimes makes people squint if they are not intent on a good photo (such as in a group situation)