The inverted row isn't just for beginners. More advanced lifters can use it to build massive forearms and crushing grip strength. Just use a thick bar or Fat Gripz attached to a regular bar.
Set up a thick bar in a rack or use Fat Gripz.
Get a good grip on the bar, pull, and pause for 1-2 second count at the top.
After 7 or 8 reps, you'll really feel it. Ramp it up by wearing a weighted vest if needed.
You probably never thought about it, but most people can't produce as much force using two bilateral limbs to perform an exercise as they can if they perform the exercise with each limb individually and then add together the force of each side. It's called the bilateral deficit and you should take advantage of it to pack on some muscle.