It kind of depends if you're "in" with the group or not. What i mean is this: there are groups of guys that you're decent friends with, you party with, hit them up to party, yada, yada...However, often times you are not a core member of the group. Kind of like an auxillary friend. You're not one of the guys they would hit up during the day to hang out and play video games with etc., etc.
You don't wanna be known as the guy that just comes and tries to pick up on their women. I was this guy. I had mutual friends with this group of guys. We were cool and everything and they were happy to see me whenever i came to their parties and stuff. However, after a while, i would always be that drunk guy that went around ruthlessly hitting on all the girls that they were friends with. Pretty soon they didnt call me up to party any more.
Social circle game takes far, far more tact and strategy than cold approaching. You have to plan two steps ahead and keep your reputation in mind, because often if you really blow it with a person or two, the group as a whole will start to reject you, especially if you're not a core member of the group. You piss off one of the guys or creep out one of the girls, and they will slowly (if not outright) start to pull away from you. And if you're in college, you have to be extra careful, because the party scene is like high school all over again. You WILL run into the same people each weekend and girls won't even give you a chance if theres a rumor that you're a creep or skeezball.
Now, this is not true with your "crew". The guys that would take a bullet for you. You can act like an a$$ and they will forgive you, just like you forgive them for stupid drunk stuff. (Obviously don't go around banging girls that your friends are dating). You live and die with these guys. You know who they are. All girls that your buddies aren't dating are fair game. And if you get permission, you can even bang some of the same girls.
You have to weigh your options. If you just want to bang her and dont care about dating her or ruining your rep with the group, then go for it. But in general, it's better to get in really tight with the group, and then make sure you're not gonna piss anyone off by going for her.
As far as these "other guys chasing the girl"... it depends how much of a chode they are. If it's a smooth guy that gets laid a lot, then he probably won't give two sh!ts. He'll just be like "damn, you banged Stacy?! Sh!t man, i've been trying to hit that for MONTHS...." and you'll be cool. But since he's been going for her for 8 months, he's probably a chode and will get super butthurt if you take "his girl". So just be prepared for that.
Like others have said, guys are normally cool about this stuff and it's probably not a big deal. But I would feel it out first if you are at all concerned about your standing in this social circle. And keep in mind that if you dont step on any toes and bang a hot chick from the group.... that will give you social proof towards the other girls in the group and probably respect from the guys.
Just my two cents
