god i fvckin love grinding. yeah you can either grab her from behind or tap her and ask her if she wants to dance. both usually work pretty well, i prefer to ask but grabbing works just as well usually.
also when you're grinding you basically are just moving a little bit to the beat, the girls not even going to be payign attention to you so even if you're not moving at all you'll still be fine lol. i've found that you can pretty much do anything to a girl whose grinding on your d1ck too, i was just at a club on sunday and this girl that was grinding on me let me grab her titties, feel her ass, touch her pvssy, etc. its real easy to hook up with a girl after you start grinding with her.
but yeah as far as initiating it goes, just ask her or grab her. there's no special technique lol.