If you guys can't do any pull-ups, utilize the negative technique...
Put something underneath the bar (such as a box or a step-stool) that will allow you to get into the top position in the pull up, where you're all the way up holding yourself up. Grab the bar, contract all your muscles to hold yourself there and lift your lower body into the air by bending your hip and knees up. Now, you should be in the top position of the pull-up...
Lower under control and very slowly. It should take you 10+ seconds to reach the bottom. It will be very intense. Once you reach the bottom, get back up the same way you did the first time by standing on the same object then grabbing the bar in the top position. Get back up there immediately and repeat.
Do this until you can no longer lower yourself under control and are therefore reaching negative failure.
(NOTE: Even if you can do positive pull-ups this negative technique is extremely useful. Strap some weight onto your waist with a dipping belt. Get the total weight (including bodyweight) up to about 110% your estimated 1 rep max. This works wonderfully for growth in general, and especially for breaking plateaus.)