What is proper bar ettiquete?

Ok I went out to the local college bar and took time to really observe. I noticed that the guys were slowly finding girls to dance with. The guys would approach them, or sometimes just start grinding up on them at the dance floor. I could not tell if these guys had previously known the women before.

Another thing I noticed was that the women who were dancing gave me no “signals” whatsoever. I have read girls who are intrested will look at you, smile, then look away. I got none. They were just doing their own thing dancing. I would think out of the many girls there, one would show intrest.

Now many of times a guy who dosen’t know a girl there (at least seemingly) just starts grinding on a girl who shows ZERO signs of interest whatsoever. She however accepts, and they start dancing.

Is it “legal” to just grind on a girl at the dance floor? Should I ask (Hey, you’re cute. Wana dance?)? Also do most guys “know” the women they are hitting on at the bar (have been aquainted with)? How does this system work?