Here’s what women in bars don’t want: A drunk dude trying to get laid. Your best move is to show you’re not that guy. If the girl is in a group, you’re going to need to involve all members. Second, approach them as you might approach another guy—i.e., don’t overthink it, and show you’re interested in talking to them, not sleeping with them.
A safe bet for an opener is always, “I’m sorry to bother you ladies, but...”
Now, the “but” takes a little bit of creativity. What you want is an interesting game or conversation or bet to bring them in on. Ideally what you’re showing is the fun you could add to their night if they started hanging out with you.
You could ask them to play a (non-fratty, please) drinking game with you, like “Never have I ever.” Basically, you want to engage them in conversation about something, then go from there. This way it’ll seem like you started hanging out more “organically,” not like you went over to pick them up.
Although, hey, there, congrats—you just did.