To perform a band assisted pull-up, take the squat rack pins and set them up at about hip level. Loop a band over each one so that the band lays parallel to the floor. Stretch the band down towards the floor and step on it with both feet. Grab the pull-up bar or the frame of the squat rack and perform a normal pull-up. The tension of the band will take some of your bodyweight off the lift and make it much easier for both very heavy lifters and new trainees who haven't built the necessary strength to perform a regular pull-up yet.
There are a few ways to either add more assistance or to make the exercise harder. The easiest to way make an assisted band pull-up harder to use a weaker band. If you go from using a green band to using a purple band you make things harder just because you are using more of your own bodyweight. If you don't have different bands you can try using an extra band to make it easier also. You can also increase or decrease the height of the pins on the rack to make the exercises easier or harder.
Assisted Band Pullups Back Exercise - Start Position
Assisted Band Pullups Back Exercise - Finish Position