If you are having problems doing Pull-ups or Chin-ups because of a weak grip simply use the Deadlift grip on the Chin-up bar. The next time you are doing Pull-ups/Chin-ups with your training partner he will be surprised that you get a couple of extra reps (that is if you were failing because of a weak grip).
That is a good short term fix. Naturally the long term fix is to improve your grip. I have found the best way to do this for Pull-ups is to perform the bar hang. Do only two sets at the end of your workout for time. Wait about 2:00 between sets.
If you can hang for 1:00 straight then you should have no problem with gripping the Pull-up bar for maximum reps. I have hung for almost 4:00 before (that really, really hurt!). Since it should take you about two to three seconds per rep, if you are going for high reps in this movement, then being able to hang from the bar for :60 is a must if you want a clean set of 20 reps or so. Naturally, the stronger your “hang grip” the more confident and stronger you will be with your Pull-up/chin-up performance.
I have found that working with the Iron Man grippers, while plenty of fun, did not help my grip for Pull-ups. I practiced until I could close the #2 gripper six times.
One additional execise that did help my grip for Pull-ups/Chin-ups is the wrist roller. You are all familiar with that device I’m sure. I used to do two or three sets after my Pull-up work out and found them quite helpful. However, the “bar hang” is the best supplemental movement that I have found to improve your Pull-up/Chin-up grip
Good Luck, with your training!