u mentioned sumthing abt grabbing a girl's arm to initiate a convo?! lol whatever u do, DONT do that, especially if u dont know her, and she hasnt even made any eye contact with u, and seems totally unaware of ur presence

ideally u shudn't walk up to random chix that u find attractive (but that dont seem to have even noticed u) to strike up a convo, she will most prolly just blow u off...go for the ones that show sum interest in u, like frequent eye contact

that said i agree with daffodil, the dance floor is for dancing, not for convo...wait till she gets off the floor, and when u find an opportunity when she's alone, go for it

then u can always say sumthing like "i noticed u dancing, u've got nice moves blah blah blah..." sumthing along those lines (and dont forget to replace the "blah blah blah" with actual words lol)

oh and most important of all, remember to have a good time..if she notices that u can dance and arent there just to get laid, that u actually have a personality, ur chances will go way up of scoring with her, and convo will flow much better

good luck