This machine can be used in a number of ways, but for its main purpose—pull ups—there are usually two different grip options: wide and close. The wider grip mimics a traditional pull up where your palms face forward and your hands are a little wider than shoulder width distance apart, while the closer grip is performed with palms facing inward and arms about shoulder width apart.
The machine is for assisting with pull-ups (as its name implies), and it does this by displacing some of your body weight. To use it, first set up the weight stack by placing the pin at the appropriate level. For example, if you weigh 140 pounds and set the pin to 90 pounds you'll be working against 50 pounds instead of all 140. You'll probably have to go through a few rounds of trial and error until you find the appropriate weight for your abilities.
After setting the weight, climb onto the machine using the provided steps. One hand at a time, reach up to grab the handlebars overhead and then very slowly place your knees or feet on the middle platform making sure to grip the handles tightly because the platform will start to lower as you place more weight on it. Once you’ve found a comfortable starting position, while keeping your body in a straight line and your core tight, use your upper body muscles to pull yourself up until your head comes just past the top of the handlebars. Use extreme control to lower yourself back down, letting the weights continue to hover before your start your next rep. If you lower yourself down too quickly the weights will release fast and crash. Also make sure to step off the platform slowly, going one leg at a time and making sure the weights return to the base before completely stepping off.