does anyone know which are the best positions to make a womens bits seem tighter to the guy, and thus give his bits more grip+friction?
Also, which positions have the opposite effect, and make everything feel less tight?
I think what you're asking is what positions are tighter, and thus give more stimulation to the guy's penis. A general rule to consider is that the positions where you are physically a lot closer tend to be tighter - for example, if she's got her legs wrapped round him, he might find that more stimulating than nookie would be otherwise.
If, however, you want something considerably tighter, then you might be able to try anal sex instead.