Picking up objects using physics alone is generally very tricky and most, if not all, physics engines aren't up to the task -- at least not with extensive customization and tuning.
The easiest way to do a grab is to not use any physics. Just test whether the hand transitions from a non-grabbing posture to a grabbing posture while it is close enough to a grabbable object. And then release when the hand goes to a non-grabbing posture. You can use the hand's grabStrength value, which varies between 0 and 1, to test whether the hand is in a grabbing posture. PinchStrength works similarly for the pinching posture.
As far as that goes, it should be fairly straightforward to implement. However, if you also want the hands to bump into objects, be able to push them, etc, then you are back into the tricky territory of working with the physics engine -- or more accurately, working against the physics engine. This is because to pick something up, your fingers are trying to push the object away at the same time that you are trying to draw it closer -- and generally, physics engines hate interpenetration of objects, so the repulsive force can become explosive. At some point you will probably need to turn the collisions between the hand and the grabbable object off so that you can hold the object, and turn it back on when you release it. But finding that perfect transition point and dealing with all the edge cases is where things can get interesting.